yeh seems like 3DS doesn't really have any killer co-op games yet apart from NSMB2.
did a little looking around and found some nice co-op games for the DS -
Advance wars: Dual Strike
Children of mana
Dragon quest 9
Well, I wouldn't say that it is worth it to buy 2 games just to play coop, but I can say I have had a blast with this game. It really depends on you tough.
Predictions for LT console sales: