The behavior being stopped is repeated POSTs to create new accounts - a spammer would instead have to GET the form every couple of hours (or if they don't realize the trick, possibly each time), something they'd hopefully do from the same IP = easier to see and ban.

Granted, if they have a huge network of distributed bots, it probably won't help. But, if they do try to register a bunch of accounts using the same timestamp hash, it'd be easier to nuke all of them at once when you discover what's going on.