Setting aside the anti-keen reading of the rules for the moment. Can anyone give me a logical reason why a "keen bow" should not be able to bestow that power on arrows?
It's hardly unbalancing, even with keen and Imp Crit, you still only have a 18-20 crit range which isn't exactly going to effortlessly slay everything you run across. In fact it isn't even as nasty as Keen/Imp Crit with a scythe (2d4, 20, x4)
I also don't see how making the points on arrows sharper is inherantly sillier than the bow making them burst into flames or be filled with Holy/Unholy/Chaotic etc... energies? Keen certainly makes sense as an ability to have on arrows.
That about sums it up for me. I can't see any reason why a bow shouldn't have the ability and it certainly makes sense that it could.