Often giving a cast time to a spell that is meant to be fast is meant to cripple the spell in PvP, nothing more. It does not help against GCD capping unless the cast time is extremely long (greater than 1.5 sec base).
Note that even longer-cast spells will still GCD cap while in bloodlust/etc. Hence cast times serve one (and ONLY one) purpose, preventing movement.
 whoops, there are more purposes: making us vulnerable (as if we aren't enough already) to melee and hence shittier as a class by allowing lockouts, and other forms of interruption.