Whilst I agree it's not a huge annoyance does it really have much impact on spam? Never having spammed a forum with links ect myself I can only speculate, surely if you were spamming it'd be using some automatic software in which case it'd just be a case of altering the time between actions to ge around this?
After posting this I now have to wait one and a half minutes to search for something else.... And 360 seconds to make this post. I can see why people find this limit annoying. Would captcha or something else not be a better alternative? I can see how the time limit has an impact on the users experience on here. I'm going to go back to google instead of searching this site directly.