Like said, mercenaries has no real story or campaign. Imagine it more like Tetris (with guns) or something like that. It's just a bunch of self contained time limited missions. Imagine it like unlocking new maps on mario kart or unlocking a new character in street fighter.
Would you wanna delete those unlocks just to go through the entire grind to unlock them again? 99% would not. The [censored] would I ever want to delete my save data on Soul Calibur, because unlocking all that [censored] is a [censored].
People saying it's "unplayable" or it "can only be played once" is [censored].
Hmmm, got it (sorry, it's like 35°C here, not the right temp to understand a second language). Still, while I can get it it's not much a bother for the type of games you mention (doesn't prevent to replay, you just replay with more choices, all the stuff you unlocked), you do concede it's a move designed to make a used game unsellable. Then what prevents them to do the same thing for another type of game, for instance, an instalment of
Okami? You can replay, but all the weapons you got stay got. Tried that kind of second-replay with the PS2 game, that got boring quickly.