The reason they don't allow it is simple - you will get quite a biased opinion. This is after all the
Anyone posting on here will be here becuase they either like the software, use it or want to use it. If I was interested in vBulitten, I wouldn't ask here, i'd ask at the vB forums where people could help.
Same with here, i wouldn't ask over vB how phpBB works or fairs.
In my own persoanl opinion - stuff like this is best suited to websites that were made for that purpose - like programmingtalk.com or something
Plus topics like these have often turned into spamfests