I'm sure there will be an instance where someone thinks up a username on a whim and goes, "I want that one," but then they have some obstacle in the way of creating it -- e.g., not being near a computer. And when they do get near one again, they forget all about it. So even if it doesn't stop the mass squatters, it may do something to lessen the 'casual' people; placing restrictions may cause them to forget about it, and if they couldn't hold on to the idea for as long as it took for them to get back to a computer, then they probably didn't need it in the first place.