I think it's better off that coop-like gametype isn't shipping with the game, so as to not take the artist's and coder's attention off of the character models, weapons, vehicles, and maps. If there was a coop shipped with the game it would be badly done, like INV is.
I think this is leaving the doors open for enthusiastic modders to make a really well-done coop gametype, with maps specifically made for coop and more quality content. INV just felt like they were trying to just get it finished, without making it the best coop experience possible, which is good, because if they spent more time making INV, the quality of other gametypes or othre aspects of the game may have suffered because of it.
I really like the coop element. That is why I play trial servers and coop trick jump.