You're assuming all the parties are getting run at the same session. Split parties are run on different nights...one night it's Pippin-Merry-Gandalf, the next might be Aragorn-Gimli-Legloas, and so forth...
That neatly solves the problem of split parties resulting in boredom for those not currently involved in the action. But it creates a couple new problems that are not trivial.
First it means that the GM is running games on multiple nights of the week. That's not an option for many people due to either work schedule or family commitments.
It also means that the players might have to reschedule game sessions based on what part of the party they are in. This also might be a problem for the same reasons as the GM's issue.
Lastly but not leastly, my gaming group is foremost a group of friends and we all look forward to the chance to get together and see each other every week. I think it would kind of stink to not be able to see some of them for an extended period of time because the PC's in our RPG went separate ways.
I don't think that just because we have as part of the "buy in" conversation at the start of the game "The party should have good reasons to be together and stay together most of the time." that it makes them a "soulless killing machine". I will say however that if the group feels strongly that they need to split up for a short time to accomplish a particular thing that results in combat, I usually will have the players whose characters are not present run some of the monsters. It's fun for them to try and kill the rest of the party from the outside once in a while.
Anyway, I've recently been reading the 4e DMG2 and, amid the other good advice in there about running games, I really like the party building methodology. Basically it's a three step process:
Come up with a blurb for your character that contains a basic personality and motivations.
Then pick a "tie" that you have with one other member of the party.
Then pick a "conflict" that you have with another member of the party (something that might cause a little friction with them, not a giant wedge that keeps the group from doing their job).
I think that this approach assures an interlocking series of relationships that both binds the party together but also keeps things interesting. I don't think it needs to be adhered to rigidly or anything. But it makes for good guidelines.