I've not played around with this yet, but I will! The function was there in EA but was buggy, at least in patch 9 which was when I experimented with it, and you had to actually swap to that toolbar before pressing the key which kind of defeated the object. Plus there was some really weird behaviour if you tried to map multiple weapons to it.
Fingers crossed this has all now been fixed as I agree this absolutely could and should be a useful feature!
(Also, moving the thread to Help, Tips and Tricks where I think it fits better.)