Why give the ability to lockout classes for Raids?

[Shamox]

When I have to pug, I intentionally give 1 opportunity for SAM/BLM. I do not want a BLM if there is already a SAM. I do not want a SAM if there is already a BLM or MNK. 1 selfish DPS per party is enough, and it's first come first serve

When you're playing with pugs, you should just be looking for good players no matter which job they're playing to be honest.