Is there a specific reason to collect all of these car keys, or all of the dog tags? To put it simply, are these only in the game for those try-hard gamers that think they have to get everything in a game in order to actually beat it? (No offense to anyone)
Dog tags in Crysis 2 multiplayer serve two functions:
e-mails only give you more detail about what is happening as you play the story.
Vehicle keys to my knowledge don't give you anything except the reason to play game once more to find all of them.