Yahtzee... Please learn your history and the overall standard for ALL Playstation consoles.
R1/L1 are for shooting, period. No self-respecting PS3 exclusive would ever map aiming/firing to R2/L2 and I for one wouldn't touch any game which did with a 50 foot pole.
If you want real triggers, go play an Xbox or Xbox 360 game, those flappy paddle things are a pathetic disgrace which doesn't even come close.
Until Sony can make a proper trigger, it's going to stay that way. However, then again, does it really need to? Why fix what isn't broken?
Any game that has R2/L2 for aiming is almost always a lazy port where the developers simply slapped Xbox 360 controls directly onto a PS3 controller.
R2/L2 is not the standard.
That said, if a minority of people want it, I don't see anything wrong with having a seperate option, as more customization options is always better.
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As for the Killzone series itself, I personally found it to be an extremely generic pile of crap, but that's something else entirely.