I could imagine them being something really of a trophy for some important kill during event fights (or the faction scenarios for example). This way, they don't lose a meaning as "trophies" like they would do if they'd be dropped after each kill. Things lose their meanings if their number is too high.
I dont see reason why event scenarios kills should be more valuable than fights during TC or in New Reno. Dont make half solutions only. Lvl 21 requirement (powerbuild only) is good enough, dogtags from same players should be stackable, of course. Look at current situation of game, there is not many reasons to play it (PvP player perspective) after one year without wipe. Collecting dogtags from enemy players should be one reason.
oh this is a wasteland and not every one is in an army or faction, maybe if you kill certain factions like ncr army, ncr rangers, unity patrol, enclave, etc. but every player? i dont think that would work to good.
(...) 5 dogtags from one player (...)
LvL 21 char is in most cases power build.