If this game was intended as a direct competition between each other, I could see an argument for disallowing ruler designer. However, as far as I am aware, the idea isn't to just beat the other players. Therefore, if I use ruler designer, it doesn't negatively impact player A and player B in anyway. It just means that I get to have a game experience that I'd like more.
If player A wants to use ruler designer, but player B doesn't, they can each choose that path. I don't see anything wrong with such a distinction. It only really makes any difference for your first character anyway. The second and third generations will be much more random. Plus, Ruler designer has a distinct disadvantage: When you use it, you lose your in-game claims beyond your starting titles. This is one reason that I WANT to use it. Mainly because if I were to play Doux of Adrianopolis, it would be way to easy for me to become Basilus of the Byzantine empire because I'm the Emperor's first son. I'd much rather, have to work into that position while still playing a decently strong duke in the empire.
Finally, with ruler designer you can name your dynasty which is always nice in a multiplayer game.
