I would say that rank 7 is pretty mainstream.
Warlock, Musketeer, Dragoon and Cannoneer are only good (in my point of view) because of high numbers of skills.
Musketeer is the biggest example of this, I managed to see A F*CKIN 85K SNIPE DAMAGE ON PVP.
But as was said before, nobody will force you to make your own build, and nobody knows how rank 7/8 classes will be on next content.
I’m planning on using only R1, R3 and R4 classes on my main build.