I don't see a single thing wrong with Bioshock having multiplayer (given it doesn't take away work from 2K's other games). "Too RPGish?" TF2 has nine classes, and eventually each one will have two sets of gear (or more) to pick and choose from. That's RPGish and it works perfectly. In Bioshock's case, you get a list of plasmids, you get a set amount of slots to equip them, and with proper balancing it'd work out.
Multiplayer is the selling point of games now; if it doesn't have it, it'd better be damn good and present hours of offline play. (Like Bioshock has, Fallout 3, J-RPGs, etc)