In a game that revolves entirely around vertical progression, simplistic AI, and trinity roles, customization brings precisely two things to the table: min-maxing with the elitism that goes along with it, and what I like to refer to as Special Snowflake builds.

The former makes the community even more toxic than MMO communities usually are already, and necessitates extremely tight tuning in order to prevent min-max builds from trivializing content, further reinforcing the social need to min-max with a code-based need to min-max. The latter basically just makes people useless and annoys people from having to carry them through content.

If you want deep customization that doesn't cause those problems, the first two have to go or be minimal factors in design. The third doesn't HAVE to go but it certainly helps.