No no no.... i didn´t mean that multiplayer is necessary. I meant said that in case of borderlands the 4 player co-op surely was the reason the game had such long legs.
I know a bunch of people that played Borderlands and neither of them played it for 4 player co op. I think what gave it legs was its artistic approach, timely release of DLC and the fact it was a FPS looting game that played well. The MP function was just icing on the cake for some.