Customization sounds nice on paper but it just doesn't work in application. Once someone finds the optimal build for anything it will be expected that everyone else follows suit.

Also, in 1.0 (pre Yoshi-P era) there was only the class system, which allowed for players to freely attach any skills they have acquired from other classes to their current ones. It basically made everyone into the same exact build with no specialization and the entire system fell apart. Such as, there were no need for the healer class conjuror to be around since everyone could heal themselves just as well as a single person could. It wasn't creative or free form like you imagine, it was horrifically boring and frustrating to do anything with. Especially since you were a black sheep if you didn't level all the things! If you are the type of person whom rather focus on one class or role.