So while reading through Houses of Hermes: Mystery Cults, I came upon the Merinita illusion mystery, Perpetuity. This major mystery allows 3 new durations that have the potential to last permanently, so long as some condition is met. Each of the 3 durations - Might, Aura, and Hidden - have some caveat that will result in the effect ending, and all 3 of them have the same level modifier as Year (+4) and all require the spell to be a Ritual, which means they consume vis. Additionally, the last statement of the mystery indicates that most (but not all) Merinita magi who learn this mystery also gain the Unnatural Magic flaw.
But why even take this mystery? By rules as written, all Creo spells can already be made permanent with a Ritual Momentary (+0 magnitude) spell.
So let's say you want a permanent spell that creates the illusion of an animated butler for your covenant. You could either use the Aura duration and add a +20 spell levels to the Ritual (and costing 4 extra vis), while also having the caveat that should the aura ever be removed from the area that the entire spell to be dispelled. Or you could just make it permanent by changing the duration to Momentary and add 0 extra spell levels, with 4 less vis cost, and not worrying about any such contingent requirements.
Opting to take the Unnatural Magic major flaw seems bananas to me, because it sounds like you're just making it harder on yourself to make permanent spells.
Am I missing something here? Perpetuity counts as a major virtue, so I would like to imagine that it's doing something incredibly powerful here, but I just can't see what it is.
HoH:MC (Houses of Hermes: Mystery Cults) p.102 Perpetuity is there, to make nearly lasting spells with effects, which cannot last for ever because of ArM5 p.79 f The Limit of Essential Nature. All Muto spells cause such effects.
So Perpetuity is one way to e. g. transform people into pigs for nearly ever, completely rewrite a target's memories for nearly ever, or get yourself big eagle's wings or an (ArM p.145) Aura of Ennobled Presence for nearly ever. It's the stuff that makes enchanted gardens people don't want to leave, and gives their rulers lasting powers there, which violate their essential natures.
In addition to what @OneShot wrote above (which shows how it is very fitting), you're misreading the Creo ritual stuff. Momentary Creo rituals don't make magic permanent, they make it real/natural when the spell ends in a moment. Then the world takes over. So, for example, if you CrIg a fire with a ritual on top of a piece of stone, it will go out really quickly unless it has some wood or something else to feed it. Your CrIm Momentary ritual for a butler won't work because you want the magic to keep on going, not some real species to just be carried away by the ambient light in the covenant.