We already do see spambots drop 5 or 10 filler comments before dropping their payload. They tend to however try and cheat by copy-pasting the posts from earlier in the thread so that they can run but not require someone to actually read the forum. Others just go for a series of "nice photo" comments in the galleries.
Also don't forget one of the first things new members often want to do is drop links to their photo gallery or blog - if we impose a restriction it hits them as well as the spambots. Whilst spammers are a pain and we do get a fair few, we also tend to remove them pretty quickly and they've not yet overloaded the mod team.