Roost usually doesn't change your type to Normal, you just lose Flying type.
Example : Skarmory uses Roost, he loses his Flying type 'til the end of the turn, so he's just Steel type, not Normal.
So, the bug can be an advantage in several cases.
Oh yes, true. It only changes to normal type when you are pure flying type.
But still, except against fighting moves (with skarmory) it's almost never positive for the roost user. (No one will hit a flying type with grass or bug moves, even if he predicts roost being used)
It's so situational, for example, i've done so many games and i think it changed nothing for me except one time (with Skarmory) against Scizor with Superpower. That's all i remember for the positive aspect.
If it's banned i will lose 2 of my pokemons :D: I'm sad it's already so hard to make a good team level 100, with the bugged moves it's even harder. :Cry: