Well, we should put it this way: Customization can work to some degree.

We see that with jobs already. After all, our jobs are a form of customization already, and for the most part, the devs do a good job making sure that you don't get shunned for picking or not picking any particular job. On the flip side, cross class skills are an utter failure in that regard. So it's safe to say: As long as the performance of two or more options remains within a certain delta value, customization can work.

The issue is that in practice, devs are only humans too and they probably aren't theorycrafting fight by fight. In fact, considering they didn't even see healers dealing damage becoming meta, they probably aren't theorycrafting at all. It is likely to assume that the more complex their game becomes, the more likely they are to lose track and control of that performance delta. And once they do, deciding which job to take might end up a decision much like deciding which cross class skill to take. That is what we don't want to happen.

So the real issue is: How do we ensure performance differences stay within a certain value without doing very complicated math every time we want to introduce or change anything?