There should be a distinction between friends and followers. I'd friend many more people, but I don't give a shit about most friend's tweets. (Most of them are about their current dinner, or something equally unimportant.) I think I currently follow around 15 people, and that's exactly right for how often I check my feed and how much time I want to spend on twitter.

So for me this works is fine, but I know many others that follow for social reasons, not because the person they start following is interesting. And if you do that, yes indeed your timeline will be overflowing all the time.

I've never not followed anyone for having too many tweets in my timeline already. And if someone starts spamming shit I don't care about (accounts with lots of followers sometimes abuse it for political or other things), I unfollow them without second thought. And I let them know why I unfollowed (like with downvotes here and on stackoverflow, I always try to comment if I do negative actions).