I like that 4e encourages DMs to say "yes". This kinda thing doesn't bother me, I'd let a player do it. People might look at him strangely if he walked around like that all the time (maybe a -2 to social skills...or +2 in some cases) and urban law might prohibit walking around with loaded weapons as already mentioned, but if this is the kinda thing the player considers cool, let him do it. He's getting his minor action (not that big of a deal) to fire a crossbow (not that great of a weapon).
Totally agree.
I think it is a kind of cool image for someone to keep dropping a crossbow and pulling another one out, it is not unbalanced, so let it happen, heck encourage it.