Customization is great for single-player games, but practically useless for MMO's. Customization in single player games is great because it enables you to use abilites for different scenarios. I use Ability A for Scenario X, and Ability B for Scenario Y. It's important because a single-player game's purpose is to change things around you and see how you act. MMO's are different. The questing phase can benefit from customization but once the quests are done, the abilities are useless. MMO's bread-and-butter are boss fights, and there is only one scenario: Tank, DPS, healer (except princess ant in Cutter, you can solo her). Customization is replaced with optimization. That becomes the main focus and your job is now to put out as much aggro, DPS, or heals as fast as you can as other exectuables proceed around you. Your customization doesn't do jack. That's why trees are useless, because you really don't customize, you're really just picking an optimization table for your rotations, and designers seldom get it right for all the classes.