I really can't see how limiting the players ammo adds anything. If a player wants to play a more "gun n' run" style then let them. That being said I do believe that there should be a mechanism to reward the player if they are able to conserve ammo - such as a guy you didn't kill helping you at a later date.
Also on the DX:IW thing would any one had minded the games faults half as much if it wasn't for the first two words in it's tittle?