The exact same problem was the downfall of SWTOR for many, many players (myself included).

Despite the fact that devs have stated this is working as intended, I honestly hope that they will change it before more and more people start to realize how 'broken' (in quotes here) this is and leave the game.

If they want the animation to play before the ability goes off, perhaps they should have just said it is an 'instant' ability and remove the 'off global cooldown' portion (even tho in reality the 'cast time' does not abide to the GCD restriction).