When I choose a class and do all its 3 circles I can now level a skill to level 15.
In my opinion the last few levels should either give the player some additional damage / support or the final level (level 15) should (for some skills) unlock one additional use-point for that skill (use-point = can use a skill multiple times without cooldown)
Please consider this as this would aencourage people a LOT more to actually do a class for 3 circles.