Its true, if you bolt the new crossmember in it won't fall out!
However!
The crossmember when welded, links the chassis legs and gearbox mounting bar. If you bolt it on but don't weld it, the loads absorbed by the welded structure are passed onto other (potentially weaker) areas. Not only that, if you are unlucky enough to crash it the chassis legs will not absorb the shock as well.
The crossmember is quite a stressed component, in some circles people reckon the reason it rusts so fast is because of the flexing and vibration it has to put up with.
Also, I saw a K11 at the salvage auctions a few weeks ago with no crossmember, bumper or radiator. The parts that were missing were on the back seat. The crossmember was still shiney and new looking! You don't think someone bolted it on then tried to tow it did they? LOL!
Nissan aren't stupid. They aren't going to buy a shiney spot weld robot to weld crossmembers on if it doesn't need to be welded in the first place!