If it starts raining mid ride, that's just plain rude
Getting waterproofs on at the roadside is a right PITA
My top tends to go on first, when can be done whilst moving, not that I would advocate that of course.
Though, I never put it on when it just starts - just in case that is all it is (often is), and by time there is enough that I might consider putting it on, im usually already wet enough as it is.
So as long as I am warm, I don't change mid ride.
Legs dry faster than trousers so bottom half doesn't get covered up, unless its below 0, sometimes I still don't bother though.