The Mages Guild stand on the summoning of Daedra was the topic of a thread I started on Beths forums because I wanted to include some real Mages Guild Policies in my Mod "Origin of the Mages Guild" and those polices had to be strictly lore based and explainable within the game lore. I asked how come the Quest "Whom Gods Annoy" suggested a Mage would be kicked out of the Mages Guild for helping somebody who summoned daedra but the Book "Modern Heretics: a Study of Daedra Worship in the Empire" said this.
"Daedra worship is not prohibited by law in Cyrodiil. Primarily this is a result of the Imperial Charter granted the Mages Guild permitting the summoning of Daedra. Nonetheless, chapel and public opinion is so strongly against Daedra worship that those who practice Daedric rituals do so in secret."
As you can see, it tells a very different story, so I asked if there was any information I was missing and the response I got from lore fans basically showed there was no agreement on the matter. One user called "abyssmal terror" suggested a theory that made sense to me, it was a simple "arkems razor" theory, eg "The simplest explenation is usually the best" and this was what he said.
"Summoning daedra, the mages guild may have concern as there are many instances of people summoning a daedra and then cannot control it, where it then terrorizes innocents. perhaps this is enough reason for the guild to interject, they do not wish that summoning becomes illegal by law, but rather under their control. the guild imposes their rules on all magic users."
"Summoning a daedra lord is a very dangerous practice, and the mages guild takes many precautions. daedra worshippers and witch covens can also summon the daedra lord, but at a considerable risk."
This was the first bit that made sense and told me Beth did not screw up, they planned it. Prior to me reading "abyssmal terror" theory, the general opinion was the problem was gaff, an error on Beths part but I felt that was unlikely and its more likely an unfinished feature. After I read that, I followed the train of thought "abyssmal terror" had started until I had all the pieces before me. Then I tried to explain them all using only "abyssmal terror" theory. With some extra reading and research I came up with this, it explains Daedra Summoning and how the Mages Guild feels about it and is based on "abyssmal terror" original Theory but assumes Beth planned it this way.
In the Quest "Whom Gods Annoy" Rosentia Gallenus summons four scamps with the aid of a special staff and cannot get rid of them. She needs the help of her friend Alven Uvenim of the Mages Guild. Rosentia is worried the people of Leyawiin may think she is a Daedra worshipper if she goes out and they see the scamps, so she is stuck in the house, this is her primary worry because as you read above, she can legally summon or worship daedra but its not socially acceptable to do so and Rosentia Gallenus is a woman with money to her name, so her standing in the community is important too her.
Alven Uvenim however is worried about being kicked out of the Mages Guild if she is mearly caught helping Rosentia Gallenus with her (Quote) "little problem". This told me that the Mages Guild does not have to catch you summoning Daedra to boot you out of the guild, they can do it if you help others who summoned Daedra too. Pretty serious stuff. Alven Uvenim actually had to do her research for Rosentia Gallenus in secret because of that.
Now in Chorrol and Bruma guild members summon Daedra right there in the Guild Halls for all to see, and do so freely without being punished. This at first does not make any sense until you think about it. Imperial Charter allowed the Mages Guild to Summon Daedra. So summoning inside the Guild is ok. The Book "The Doors of Oblivion" tells of Daedra Summoning in special chambers by powerful Mages with a second mage helping to control the Daedra that got summoned. In the Book "On Oblivion", one of the most powerful conjurers enters the Oblivion to do research and never returns. The dangers here are very very clear, the two books explain beyond all doubt that summoning Daedra summoning is dangerous and the book "Modern Heretics: a Study of Daedra Worship in the Empire" tells how the people do like like Daedra Worship at all. It all tells the story of secret studies of Daedra. Some more controlled and secreative than others.
Knowing this, it all starts to make sense.
The Mages Guild would not want people trying to Summon Daedra out side of the carefully controlled surroundings that each Mages Guild Branch offered. Anybody caught trying to summon Daedra outside of the Guilds grounds would be banned. It is also clearly forbidden to help anybody who attempts it, as Alven Uvenim case shows. And the Bruma and Chorrol branches prove summoning inside the guild is well within the rules.
The Book "The Doors of Oblivion" describes a chamber that should actually be used to Summon Daedra and even these sounds a little overkill compared to what we see in the game when Daedra are summoned, so the game it self has made light of the situation, making it appear less dangerous. But all the background info shouts out that "Summoning Daedra is dangerous and the Guild does not allow its members to do it outside of the Guild own Grounds" for the sake of the community it serves.
Even allowing for some vague refences that appear at first to shoot down this claim, like the Thieves Guild quest and the Daedra Summoned to deliver a message to Hieronymus Lex, you have to remember it was summoned by the Council of Mages to tell him to move his guards back. So these were not Novices and sending a Daedra to deliver that message was not as simple as waving a hand and shouting "appear". The Books shows its hard because the Daedra needs to be controlled and for the Council to make the effort to do that meant that Hieronymus Lex would get the message without even reading the note. They were pissed and he knew it.
So it appears the Mages Guild have strict Guildlines to the summoning of Daedra by it's members outside of the guild.
One other odd question remains, Aylied ruins like Ceyatatar have mages in them and those mages summon Daedra and attack you on sight, they wear the uniform of a Mage from the Mages Guild, but no other faction in the game that uses a Uniform EVER shares their Uniform with another faction. They all at least have clothing with a different name. For example the Black Robe and the Dark Brother Hood Black Robe is the same Robe, but in the game the name is different and one is enchanted.
As you can see, Beth made a big effort to provide unique uniforms, so what is the chances of those Rogue mages being the only faction in the game that just happens to share the same uniform as the Mages Guild ?
I think its ZERO, theres no chance it is an accident, "arkems razor" suggests the simplest explaination is the right one and that means if they look like Mage Guild members, cast spells like Mage Guild members but clearly breach mage guild member rules right in front of your own eyes. Taking in to accord what Alven Uvenim said. Then they must have once been Mages Guild Members.
Some disagree with me here, but they have yet to explain why they are the only ones with a uniform that is already used by another faction. I think there is a link and that the link is they are Daedra Worhshippers who joined the Mages Guild to study Daedra and got caught summoning Daedra or helping others outside of the Mages Guild grounds to Summon Daedra and got booted out of the Mage Guild for their troubles.
This makes sense of the whole Daedra summoning issue, it explains the Quest "Whom Gods Annoy", it explains the book "Modern Heretics: a Study of Daedra Worship in the Empire", it explains why those Rogue Mages were uniform of the Mages Guild but attack the guild members and everybody else whp approach them. It explains why their chosen creature to summon is always Daedra and not a normal creature like a ghost or skeleton. The Uniform says they were in the Guild once, the Daedra they summon tell us why they got booted out and the game LORE tells us all about how the people feel about Daedra Worshippers and Summoning generally.
All the facts fit.