Tribes 2 was quite unique in that there were a lot of point-based incentives for playing as a team, with things like defence in particular being well rewarded. This is a concept surprisingly few games have caught onto, although Team Fortress 2 and Battlefield are notable examples.
I played TF2 a lot and I saw lots of fantastic teamwork, although it really depends on the server. If you're a regular and you play with other regulars, typically you work together very well. It's also quite well designed in that there's not so much incentive for defending in itself, but defence-orientated techniques can be very good for your score.
Battlefield: Bad Company 2 (which is temporarily de facto Battlefield 3 on the PC at this point) is good too, as really you're screwed without working together and again there're plenty of incentives for helping your team.
So no, I would say team-based gameplay is pretty much as strong as ever. Really players have always been selfish and it's up to the game designers to leverage that in the right way. Tribes 2 was just a particularly
good example of this and I miss it dearly.