One thing to consider... valkyrie, zenmaster, and assassin are not "max roles". They are not graduation roles to any particular class. They are special classes, and when you're in one of those classes, you can switch to another when you see fit.
I agree that two of them come way late in the game, and I wish they arrived earlier. I am thankful that they made the game replayable, or else those two special classes would be close to completely useless.
I will also mention that your zenmaster or valkyrie can easily be 20th level by the end of the game, even though you make the ascension in the last dungeon. Such is the power of experience...
