The only people who create usernames longer than 50 characters are vandals. Super long usernames screw up the formatting in lots of different UIs and are a pain to deal with from a design perspective. The longest legitimate username I know of is User:Chase me ladies, I'm the Cavalry, which is still only 32 characters. The current limit is over 200 characters, which is totally crazy.
What Wikipedia looks like with a crazy long username: