Personally I think the 2.5s GCD exists so that healers using a controller can compete with those using a mouse and keyboard. If the GCD was short then 'tabbing' through the party list with the controller would likely put the average user at a disadvantage compared to a mouse and keyboard user. I find the GCD pretty excessive as a DPS since for the most part I already know what I am going to press next and this results in spamming the next button more than I do on other MMOs but maybe that's just me.