I am definitely in agreement with this idea. To refine it further, how about whenever we are offering on someone's pokemon, we have an option of "Legendaries only" and "Non-legendaries"?
It would simplify trading by quite a bit if we had this option, especially since right now we have to scroll through all of our pokemon before we can offer. For someone like me (who has over 400 pages of pokemon), this idea of being able to look specifically at all of my legends is very appealing.