You know what sounds a lot better in theory than in it is in practice? A spork. "hey look, it's a spoon and a fork rolled into one!" You know what? You get an inferior fork and crappy spoon that way. I just want Obsidian to build the best fork they can and leave the spork to somebody else.
An interesting argument, but I've used some awesome sporks, and moreover they cater to a group of people who find them immensely useful, such as (as I will refer to them) outdoorsmen.
But if we can ground this back in the realm of video games, and specifically the IE games which supported multiplayer, I contend that the theory and practice are both wonderful. Several people have already mentioned the fond memories they have of playing these games with a group of friends, so I won't beat that end of the dead horse, but I'd like to hear some specific reasons that people seem to think adding multiplayer would ruin a game like this, preferably with examples.
The argument that I recall as I'm typing this is about how dialogue would be a problem in a multiplayer mode for this game, which I find to be nonsense; do you have the same problem with dialogue when running a Dungeons and Dragons campaign, or during a night of Rune or Pathfinder? There is really little difference between the two, except that the video game has to, by necessity, be slightly more restrictive in choice and speaking order. But if you can successfully play a pen and paper RPG with your friends, then there is nothing that I see stopping you from enjoying a multiplayer mode of this game; in fact, it's hard to play D&D when your friends are on the other side of the world, but having the ability to use TCP/IP connections means that I can play BGII with my brother while he's deployed in Afghanistan. And if the multiplayer development hurt some aspect of the game that I was completely ignorant of because I was too engrossed in the art, the voice acting, the game mechanics, the story, and the wonderful memories of conquering bosses with my brother through what must be now at least 50 playthroughs of varying difficulties and arbitrary rules (like the Bhaalspawn must be a dwarven kensai specializing in spears - go on and try it, it's pretty fun!), then I'm pretty sure I'm glad they took a hit on some ancillary part of the single player experience to bring me everything else.
Note : I tried to not beat the dead horse and failed, twice, so I'm leaving in my little rant. Please excuse it on the grounds that we're all just passionate about this game, and want it to bring back a golden age of computer RPGs.