See also:

http://news.ycombinator.com/item?id=2368486

(there are lots of comments, a couple of months ago, it would have been easy to see which ones are "interesting", now you must read them all).

You are absolutely correct. Timestamping alone does not suffice, but it is another layer of defense which does work. Most bots do not have random delays.

I'll try to look up some numbers on this.