I actually hate using R2 as the fire weapon button. So much in fact that I used the classic control layout for GTA4. I typically don't play games that don't allow me to use the R1 button to fire the weapon in 1st/3rd person shooters.
i am the only person in my group of friends with a PS3 but everyone i know has CoD4 so one day a few of us planeed to get together and have a tournament a few days after so i switched my controls to flipped on CoD (r2 to shoot) and i havent used R1 since.
so the thing about the r2 triggers for shooting is because the way the analogs are placed and the shape of the "triggers" themselves. I envy the 360 controller. It was like it was built for humans. How ever the ps3 controller was rushed. you know i wouldnt of mind the boomerang controllers cause they actually looked comfortable.
Anyways.....ya the reason why i dont like the L2 and R2 pedals for shooting is because of the way the analog is placed on the controller. Having R1 and L1 is way more productive and comfortable since its closer. when the 360 controller has the left analog on the top of the D-pad so it was initially made for using the shoulder buttons easily. ALSO the L2 and R2 pedals on the ps3 controller arent shaped like gun triggers so they slip off easy since they slant when fully pressed........not a good thing for the player shooting his weapon.
Playstation fans like myself will always use the r1 and l1 for shooting because we used it ever since PS1 came out and i feel most comfortable with it.
thats my 2 cents on why they should always have r1 and l2 to fire.