Depends on the game, dunnit.
In FPS games with iron sights, you have to hold down the right shoulder button, the press the right trigger to fire. You can't do that with one index figure alone.
Mostly, my index finger rests between the L1/R1 and L2/R2 buttons, pressing them as needs be. Apart from when I played on an XB1 and that was impossible and my index finger ended up pressing both at the same time.
But seriously, every since the PS1 and its two rows of shoulder buttons, some games mandate both index and middle finger usage, as you have to press combinations of them at once. You can't use index for everything. It's utterly impossible.