We have a breathing idle on a character, and a collection of "breaker" (Ex.: Character picking his nose) How should we implement something like that while not making any script/class that would drive the animation state? No matter how I looked, Mecanim doesn't appear to be able to pick a random animation in a list without external help.
+1. That's something that I miss from Sage, which was basically Mecanim for the Legacy animation system before Mecanim was out. When Unity 5 introduces scripts on animation states, it should at least make the external help a little more modular and manageable (e.g., put a "random transition" script on the idle state).