There is an added bonus that it is harder to link related attacks and find out which group is behind which wave.
Also, there will always be some amount of legitimate users with random names - and any other easy to generate will-always-exist pattern would fall into collisions more often.
Relevant username? It seems that the spambots are learning at a geometric rate - I give them a few weeks before they go FOOM.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.