That's a fallacy. Monks aside, having 100 SS (going from 2.5s to 2.4s GCDs) will enable you to finish the last attack that you would have otherwise been unable to execute, [4.2% * # of prior attacks] of the time.

E.g. if you have 10 attacks before the boss mechanics on your head, then there's a 42% chance that you gain an extra attack while leaving the boss due to skill speed.

It's funny to see people demand a 100% chance of a full new GCD from skill speed before it's worth anything, when they value crit so much even though it only gives a 7% chance for half of an attack.