This I found is still debatable since I haven't seen many talk about it so i'm deciding to make a poll.
I ultimately feel that if GB decides against playable Aliens in ACM MP (They haven't announced it yet but we can assume that its in BETA since GB demos it at PAX) in favour for scripted aliens i would sincerely respect them forever for it.
That doesn't mean you couldn't make a great multiplayer- just without playable aliens. Hense making it more diverse like the scripted events of the Single Player Campaign.
Consider this: SP has a drop in drop out which could be touted as MP but scripted.
I found so far through watching the PAX MP videos that the aliens don't quite do a whole lot more than what we've seen in AVP 1999. If being able to play as aliens removes the ability to place more events in place via AI programming (ie, coming through vents and through the ceiling, through the ground, close encounters) I would certain vouch for this decision as the best action to take.
I want to source this as reference
Batman Arkham City:
This game was quite possibly the 'go to' game for me for a several months. I hope that all games can be executed this nicely in terms of balancing gameplayer controls and fighting, side quests and canon, unlockables, and challenge modes.
This is a quote from BAC wikipage proving that drastic change can favour the game overall.
"The presence of multiplayer had been rumored shortly after the game's official announcement, but was denied by Rocksteady Studios.[44][45] The developers had considered adding a multiplayer element to the game, but ultimately decided against this. According to Hill, they asked themselves "If we use all of the energy that is required to create multiplayer and instead focus this on the single player, would that deliver a better overall game?", and felt that they could not succeed in this, instead using their efforts to strengthen the single player game"'
*I am merely quoting Rocksteady's development decisions. Its not a request to make no multiplayer in ACM. just no playable Aliens in ACM.