That's really just to prevent accidental double-postings by real humans... done more because in the past the site was slower and people would often click once, wait a half second, and click again thinking it hadn't gone through. You'd also be surprised how many people simply double-click, ending up with unintentional duplicate postings.

There's all sorts of limitations I *could* add that I don't see the need to... like preventing a poster from making more than one reply in a row to a thread (saw that one somewhere, found it annoying).