I have to say that I never liked that feature, and had even asked if there was a way around it. Here's why: sometimes I want to click on the "Submit Reply" button and then immediately go somewhere else on the web (to a new page or a different topic) or even turn off the computer. When I did that, and there had been another post while I was typing, my post would be lost. As a result, I'd have to Submit, then wait to make sure I didn't get the warning, then click again.
IOW, I like to click the Submit Reply button and know that no matter what, my important and insightful stuff got onto the board.