Too bad in the end everyone pretty much is the same anyway. People find the strongest builds and everyone copies them.
yes, assuming every build fulfills the same role. in XI, if you were a dark knight, you were going to be a damage dealer, regardless of your sub, therefor people would find the sub that allows them to do that job as effective and efficient as possible. Same goes for White mage, they will always be a healer, no amount of customization is going to change that.
But in a game like WoW, most classes can change entire roles based on what they have specialized in. a warrior can be just as effective as a DD as they are in a tanking position. priests can spec to heal or do damage, paladins can spec for healing, damage, or tanking, and so on. i havent played the game in a few years so some of this may have changed, but the last i heard is that theyve actually increased the diversity in classes.
so yes, it is possible to have customization without having a single overpowered build.
cookie cutters within those specific builds however are a different story.