As I mentioned in a sibling comment, a big part of Twitter for me is meeting new people. Not only have I had good, thought-provoking conversations on Twitter (for all the 140-characters jokes I don't find it that difficult) but I've made friends who are now RL, in-person, at-the-bar-on-Fridays friends there. That doesn't happen when you're blocking external folks in that way, and that's a bad tradeoff to make just because "well, those people are dicks, we can't control them, emojishrug!".