Well the intent is it allows people to catch and correct their mistakes, but also preserves the original intent of the post. There have been instances where a debate breaks out and someone offers a rebuttal. Then the OP goes back and changes his post claiming that's not what he meant or never said that. When spirited debate happens and both parties are actively posting 30 minutes is a long time. I mean I could bump it to 60. If such intellectual dishonesty was to ever happen again I could easily look at the change log for the post.