Ok, for starters.
Why we need killshooter; simple really, stop the dweebs and n00bwads going "ye4h!!!111 kill green d00ds!!11 i b super cool"
Why we can't have a system where the bullets don't do damage to freindlies: Ever played DoD with FF off? You'd find most players don't give a flying **** where a freindly is unless he's passing through a door way and therefore 'blocking his l33tness'.
In summary, killshooter is a necessary gameplay consestion because there's alot of smack-tard-dweeby-arse-n00bwads out there with access to daddy's credit card, and there's even some that manage to get credit for themselfs amazingly.