I think that's pretty much what ppl said above that 2.5 seconds won't let players pressing buttons too much while it's not that slow.

one more thing I think can be contributed to the 2.5s GCD is one point from dev team's interview @ 1.0 to 2.0 period. There was one thing they brought up about the graphic quality and game setting. A pretty game like FF14, dev team dont want players kill mobs too fast. if players killed mobs too fast that means they need to make more mobs on the field. So, the dev teams default setting in their mind is 15-30 seconds a mob. From there, they setup the dmg per second on combo damage or AoE damage. If you are in dungeon, you can feel roughly each group of 3 mobs takes you 1 mins to kill them and run to next bunch... I know that cus my divine seal will always ready from batch to batch