I see my arguments made some surprises. I may have put them in a way provocative: I do like to provoque discussion, for the betterment of the game. (Unrelated to this, I play a tytalus for some time now. 
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But I try to make statement only if they are true to me, and this is the case now.
I'll try to get more precise on what I have to say, and also answer some good arguments that have been presented.
What I am critisizing is the adding of mystery cults to the core rules of the game. My statement is that those "add-ons" (= most mysteries) have been brought into the game in a certain way (= mystery rules) , and they (the add-ons) would have been better made in another way. Namely, within the core rules.
My argument is that the rules about mysteries actually restrict the game, more than really adding to it.
A good counter-argument that was brought here is that the Mysteries actually add to the game background, and actually add more options to the players.
Never was my intention to say that it wasn't the case. I also don't think that is what I said, but this is not important.
The point I want to make, is those background and options should have been included in the core rules of the game. I mean within the 5 techniques and 10 forms of hermetic magic. My main argument here is that there is no other rules needed to add most of those backgrounds and options for the game.
So, what I say is that the mystery is of less quality than an hypothetical replacement using the core rules. I'm reminding you about the importance of JeanMichelle previous statement about complexity of the rules.
Don't get me wrong: I am very thankfull of AG to produce all this material. What I am doing now is express what would be a betterment for me, and why I think it could be good for most ArM gamers.
Here is why and how I disagree. First, this is the real question. Because any adding to a game should be judged by the results it yields.
I've already said that it is my opinion that there is other, more "profitable" ways to make new addings to the game.
Now, I don't want to say either that those who like to play with the current mystery should not, or anything near that.
What I say, is that it's possible and probable that some people will prefer using the simple and better "proof-played" core rules.
It's here that mystery restrict the game options, because when some power is included into a mystery, it is no longer available to "core rules magi" (vannilla mages).
Here an exemple to illustrate this: the feary mystery have the duration "Until". This would imply that I need to get into that mystery to get that duration. And there are many rules about doing that. (Otherwise you need to change the idea of what is a mystery)
Another exemple that astonished me, is in HoH:S, p.113. The Colombae can use AC to boost penetration to wards. This mean my character, a ward specialist, cannot use them for his wards !!!
This is not a Mystery, but the critic is the same, and goes for anything that impede upon the core rules.
In my opnion, if it can be done with the core rules, it should. This enable character to seek powers more easily, it's less restrictive. I prefer to see a complicated story about getting that power, than complicated rules about what have to be done and sacrificied to gain it.
This is also a good exemple of problem that can rise from this whole idea of getting into alternate rules (non-core rules). I've said my character was a ward specialist: so I would like to make wards against things like Mental control, PeCo, dispelling, etc. While there is actually no guideline to do those (and those guideline is within core, hermetic capacity, proven by the working of a ward against demon), they go on an entire page recopying the guideline already present into the core book ArM5 (p. 111) !!! Instead of having a description on what Colombae can do that others cannot, I would like to get things that most character can do with their hermetic magic !
The final result is: Vanilla mage have less power than described into the core book (now they can't use AC with wards)
This is the same with most mystery: to know what a simple, hermetic mage can do, you no longer need to simply know what Muto, Terram, etc means. You need to know what all mysteries can do, because if a character happens to be wanting to do something similar to one of those mysteries, you have to know that a mystery exist for that particular power.
A storyguide could decide that to make a room be larger in the inside needs MuTe for a stone chamber, with a Base level of 20. It's not completly illogical. It's not a power unbalancing. It could be made a new guideline into a supplement book. But i've learn on this forum (serf's parma) that there exist an "Architecture mystery" that could do that. Damn. Now vanilla Hermetic magus can't do that. One less power for them.
My other argument is about how restricting to adventures mystery is.
That is what I meant by saying: "Being in a mystery cult is really a restriction as to what interesting adventure you will play. "
I meant : "being" as in "being active about the pursuit and use of a mystery"
I didn't meant to say that it restrict a player's option. They restrict adventures, i.e. actual playing.
If you play within a Mythic Europe with Mysteries, there are certain things that can be acheived only in certain way. If you have a character actively pursuing a mystery cult, you will need to play those stories about mysteries. This can also be fun to certain players ! But I would have prefered it to be something more like the Story seeds, not implemented into the rules.
[code]
e.g.:
You could have: "The texts about how to make [insert hermetically described power] is kept by the three main lines of Verditius magi. They don't share it with outsiders. But, there exist a rumour of a researcher who made similar discoveries in the 10th century. All his researchs paper are now lost, and there are different rumours about where they are today..." and, as with original research, you could get that novelty simply be spending season reading those books.
Or, as an alternative, the troops could decide it is available to most magi, because it's of general interest (like the Adaptive mastery ability (HoH:S p, 34) : why the hell do one need to be into the Cult of Mercury to put xp into that ?!?)[/code]
Now, imagine a Mythic Europe without mysteries. Most powers can be achieved within Hermetic magic. That same player could be pursuing the same powers as he did with the mystery, but since the power is described with the core rules (a new guideline or a new duration, as a strong and simple exemple), there is no time lost learning new rules. You get straight into the adventure.
True, but he can, he could, he has the option to...
In our saga, our alliance decided to buy a text book of Boots of teleportation. Our verditius magus bought one filled with verditious runes. My tytalan magi was really pissed out about that. I ask you: Was he rightly pissed out, because he couldn't use - at all - that text book, or was he just a weirdo, because every mages acknowledge that verditius are the best for making magical items ?
Really, this was ambiguous, and yet today, I can't roleplay about that because it isn't consistent with Mythic Europe. Yet, I can't see why my magus should be happier to have that text book filled with those pesky runes. Is that text book better than a simple, core, vanilla, hermetic text book, I ask you ? Should RAW verditius mystery be praised or shun by the Order ?
I would add something about the "munchkinism vs roleplaying" debates, but this is already too long ! I also don't think it has anything to do with this discussion.
Sorry for the long post. 
