Most of us enjoy games which emphasise teamwork. I love Tribes 2 and Day of Defeat, which are both highly orientated towards teamwork.
However, as far as the MIGC is concerned, they are too complicated to "dip in and out of", as Huz said. Straight deathmatching where your objective is clear and untainted (even Worms is plain old deathmatching, except in a turn-based fashion).
We also generally only play with about 3 or 4 of us present at a time, which makes games like that which rely on more people being around just not work.
I have no doubts that ET could be good (although graphically it looks very manky, and like a poorer version of various other teamplay games/mods out there), but for our purposes it won't work.