As it's already implemented, and I didn't see it, it is irrelevant, just a suggestion.
For those who are wondering the benefits,
It's a simple, eliminate one click. And allows for easy see what everyone else said before you finish replying. It also helps when moderating a forum, or replying to multiple threads extremely quick.
I help on many forums, and I instantly look for the quick reply, as it's fast. Also with a bit of tweaking, can be done via ajax(a javascript construct),which reduces it down from 2 page loads, to 0 page loads.