This argument about theorycrafting is valid, but it has its limits. It would certainly be prohibitive to allow broad, free-form customization--i.e., Freelancer/Traveler would be impossible--but that doesn't mean customization is impossible. The issue is making the choices powerful enough to be valuable and restricted enough to be competitive. Suppose, for instance, you're playing a healer and have to choose between Swiftcast and Cleric Stance. You can equip one (and only one), and you can't change once the duty starts. Some fights might be easier with one or the other, but you're ultimately trading a very valuable cooldown for a significant DPS increase. I suspect even among a relatively small sample we could get disagreement on which is more important to a healer. It might be skewed, and high level play would likely default to one "optimal" option, but the choice would by no means be illusory.