ye rbgs have certain "comps" but in RBGs it's much more important how good your team mates are.
You can play the best comp in the game but if for example your lock doesn't know how to peel for the healers and how to setup cross-cc you would be better with another DPS.
In RBGs hunters are considered shit but nonetheless I was able to be in the top20 in RBGs last season because I made my class work.
I was almost 300 rating above any other legit hunter because I knew what the my team needs and could deliver.
This means abusing Knockbacks, exotic defense strategies, winning 1on1s easily, peeling for your healer and FC.
Yes RBGs are comp based but those comps are based around 8 classes and the rest can be replaced by another one (random DPS-slots).
With all due respect, what you said is true word by word for arena