Quoted:
The crimp is to keep the primer from falling out and jamming the action, when you have someone shooting back at you this cant happen but for the civilian shooter it's best to not have the primers crimped, that way if the load is to hot the primer can come out and make the shooter aware of it.
Specifically, crimped primers prevent jams in automatic fire. This is a MIL-Standard that is applied to all GI cartridge ammo (not shot shells), even GI .38Spl. But as EWP points out, up I cannot use some primer indications of pressure with crimped-in primers because the crimp hides some indications and changes others. Further, I think that if there were a real use for handloaders to crimp their primers, there would be a dozen products on the market to do this, and we'd see "oh no not this crap again" arguments over which one was best.