I am one who believes teamwork and cooperation to be core elements of D&D. A pack of loners who never help each other out isn't my experience. A game where players coordinate with each other to overcome challenges is.
That said, teamwork only functions when it is an option. If everyone is jumping off that bridge, I must be able to go my own way as a player.
However, I also strongly believe it should also be in my best interest, if the rest didn't jump off that bridge either. And being able negotiation between players on what to do next and how is a huge part of that. Players unwilling to interact is going to shut any game down.
==
Another element I think Monte may be overlooking or simply didn't mention is class scope. It's almost status quo now that D&D is primarily about skirmish combat. Classes can have their particular areas of strength and others where they are weak, but I disagree this makes all of them mandatory.
I'm not talking about the RPS of artillery, cavalry, infantry, or 4E's Roles. What I mean is the scope of the adventure changes depending upon the scope of the character classes chosen by those involved.
Fighters excel at combat in all respects, but wizards are far more about magic accumulation and experiment. Thieves and sneaks are almost always attempting to avoid face to face combat too, but are great at accumulating monetary wealth and power. Clerics are almost a forgotten class relegated to spot healing in combat, but where they really shine is in dealing with intelligent creatures.
Not having a wizard does not mean the rest of the team is dysfunctional. It means that magic is a more difficult avenue of exploration for that group. It's not that the scope of magic is off limits, but that in their adventuring and in the adventuring world magic is not as accessible as it would be if they had a wizard on hand. Can they still pay one? Sure, but money is a poor substitute for another member of the team.