I thought it interfered with the overall interface and was redundant and i hate redundancy. If you want to replay, click "Reply" and wait 1 second for that page to load. Reply. tada.
On a lot of forums the quick reply won't be necessary, but on these forums I think it's quite nice for a quick answer: "yes, that'll work" or "wow, great job but I'd change this or that"
I thinks its quite useful. For simple replies you have it right there, type and go. If you neeed to post code or long messages or extensive formatting then you use the regular reply.