Two words: Suicide Rushing
The Alien class allows me to be a dretch so I can suicide rush on any human I want!
Wait.
No.... you couldn't be saying dretches are great for feeding are you?
Because...
well...
If you are that's just
dumb. Aliens are the strongest race in Tremulous. This is because they are fully capable of performing everything from camping to rushing - with proper coordination and communication - well.
They have a class to fit any role, be it camptacular hallsweepers like tyrants.
Mobile healing/opportunist/guerrilla units like Lisks.
Hit and run/base-breaker/harassment units like Maras.

Or highly-mobile assault units such as Dragoons and Adv.Goons.
And their basic units are almost broken in stage 1, somewhat dangerous in s2, and annoying as hell in s3.
Plus there is never a need to return to base to change class, making even a lowly granger capable of killing a handful of surprised humans...

Aliens are the best race when used by the right people.