Yes, it happens all the time, and I don't let it get to me. Obviously, I don't bother following them back.
I can't speak for twitter but, when I have reported accounts for abuse, they have been pretty swift in disabling them. Of course, it does depend upon the type of abuse. Just following someone else is not nearly as bad as outright spam in the timeline.
What annoys me is that the spam accounts do not instantly vanish from my Followers list(s) after I report/block them.
More annoying is that spambots get accounts in the first place, of course. Numerous of the multi-account follower requests I receive hit at the exact same time and the profiles contain similar, erm, action words. How difficult can it be to pattern-match when you've got Twitter's capability?