Not meaning to steal your thunder, temple. ;]
I think that the use of the word teamwork is hardly proper when referring to the alien team. It is assumed that the aliens should be able to work as a collective. This assumption is correct. But how they work is completely and totally different than how the humans work.
Humans are most effective in groups, as, like temple said, they have a strong ability to focus fire. Aliens, however, are largely better off as soloers. This is due to their low range and high health. Aliens require coordination and teamwork at the enemy base, but (barring the first three offensive classes) they are very effective reaching the enemy base alone. Humans, however, require teamwork to reach the enemy base (depending on the path that they take, the size of the teams, and the player-skill-stack).
I feel like a lot of the changes in 1.2 try to force teamwork on aliens to reach human base which, really, shouldn't be too much of an issue. One of the things that drew me to Tremulous originally was the fact that tyrants were terrifying. So were goons. Hell, so were maras! And this was all solo. I loved Tremulous because it was great accomplishment when my buds and I took down that rant. It was an accomplishment when we heard that fuckin' 'goon moan for it's little life. And man, aliens were fun as hell to play! Each class had a very distinct role and a different style of play. Now, they still do, but they've been all jumbled up and switched around and maybe, even, a little bit broken. Don't get me wrong now, I've had a lot of fun playing humans on GPP, but aliens? I guess they just don't make 'em like they used to.
There's something about them now that simply makes them ineffective alone (could it be the nerfed attack speed?). Against turrets? Yeah, you should be switching out snipers and rants. Hell, against a persistent group of advancing humans, you should be switching out your snipers and rants. This has always been true! But against a group of two or three? You earned that fuckin' 'rant. It should work well.
Of course, this whole bit could be discounted with "all classes are great if you master them!" and yeah, I agree, they are! Always have been. But much of these changes have made aliens not restricted by skill, but by the game's coded rules.
Odd formatting to make this seem less like a terrifying wall.