I don't think a single person nerd raging about RE: The Mercenaries has even played the game. I'm guessing most don't even currently own a 3DS.
Whether Capcom uses this "one save" idea for other games remains to be seen, but it has ALREADY been used for SSFIV 3D. It keeps one save file and as far as I can tell, there is no "reset" option within the game. I don't see anyone claiming this hurts the resale value of the game either.
"But RE is a story based game!"
And that's where actually having played The Mercenaries would make the difference. It isn't a story based game. In fact, it's essentially a series of time trial challenges using levels and characters from RE games with absolutely no underlying story at all. In fact, Mercenaries would be right at home as an arcade game where the player is essentially playing to achieve the highest score/fastest time on a series of levels of increasing difficulty.
Forget the save system; it's really not an issue unless you're under the impression that everyone who plays the game on a single cartridge is entitled to their own "Top Score" list, which is essentially all that the game save is.
The real issue is that Capcom is charging $40 for a game that is essentially a time trial mode for a proper, full length RE game.
While it's true that the 3DS continues to lack in games, most 3DS owners should probably save their $40 for RE: Revelations if they want a real RE game on the 3DS.
And yes, I bought the game and have actually played it. A little disappointing? Yes. But it has absolutely nothing to do with the save system.