Henners, I still have not the impression, that you fully understood the idea, so please object yourself

1.) first lets the team pool out, I dont know how stof wants this to work.
I initially brought this in as a replacement for the kills required to stage up.
The change I wanted here is also possible right now: if you need 20kills to s2, then allow only 7 kills per player to be counted so one high skilled player will be forced to teamplay. (this is not a requirement for an exp system though)
2.) runaway win:
you like to bring up the dretch vs. battlesuit example, but this is like stof pointed out braindead, since its like 0 kills vs. s3.
and now the 3year old version of exponential exp for you:
- dretch gives 10exp
- marauder gives 100exp
- armor needs, 50exp, $decent_weapon lets say another 50exp
- s2 needs 1000exp
if humans get advantage in the beginning of the game, they will get armor + lasgun fast (10 dretches)
but if they want to reach s2 and aliens suck, they would need to kill 100dretches instead of 10marauders, which gives aliens enough time to catch up, since marine with $decent_weapon gives 100exp.
by the time humans reach s2 there should be at least enough marauders to keep them of raping alien base - if there are not there is a major advantage on human team and the game is lost anyway.
and if you finally understood this (you may read several times over it), dont hang up with the numbers; its just an example.