I would think ideally the single player and multiplayer games should be identical tactically, i.e., if turns are simultaneous MP, they are in SP as well. Length of game is an issue, but not if you play medium maps.
And while a community might be nice to look for players, I think most people will end up using the MP function for coop play in something like the Altarian Allies (I think the name is, from Altarian Prophecy where Terrans and Alterians fight side-by-side against a universe of hostile aliens). People will likely also play 3-way free-for-alls, but I doubt many people will play 5 person games. So, these are more like scenarios in AP and not like traditional games, so that allows for more reasonable length games and people will likely play with friends and not need to find people over the Internet (though the ability to find new people might be fun).
Finally, I really think GC2 needs MP. While GC1 was a great game, it is just a lot more fun to play with friends than it is to play alone (esp. on a long game like GC where you can easily disappear for hours and hours). So I really hope the developers take the time to consider adding MP into the base game (it also sells more games--if it has MP, all my friends will buy it since we like to play together). Even if it is an add-on, it really has to be basically designed before the game is done so that it is basically an identical tactical game--if it is turn based in SP and simultaneous in MP, it just won't work (or something similar). I am not in love with simultaneous or anything, but it seems to be popular here. I am fine with order options like in Stars! (which is more in keeping with GC anyway) that allow you to issue standing orders to your fleets.
So end my thoughts, but I would like to remind everyone how cool it would be to release your new theoretical deisgn on your friend's theoretical design to see what happens... around here, we are already thinking up ideas based on the limited state of the beta, so I think it is safe to say multiplayer is something that we would use extensively. You just never had that custom ship overlay in GC1, and so, the desire for MP is just stronger in GC2.