Canceling a GCD has no impact on spell-speed effectiveness, you get the benefit of Det, crit, or SpS the moment the spell goes off. Having faster SpS also lets you start slip-casting faster.

The issue is that people are not taking all factors into account. Having 2% spell speed means you have a 2% margin of error in terms of getting the max GCD's off during something like trick attack. I'd love to say that everyone is pro enough to time their GCD's off exactly to get in the maximum number of GCD's during TA, but in reality stuff happens or the ninja accidentally delays his trick attack by .2 of a second, causing random person x to miss the trick attack window.

Sure, .05 seconds off GCD is an extremely small amount, but so is the equivalent 2% increase to damage from determination. If there is a single situation in 50 GCD's where that 2% faster cast lets you get off a cast where it would have previously been cut off (or misses a buff window), SpS becomes equal or better than Det. This would only need to happen 2-4 times in a typical long boss fight.

It's easier to see the benefit of 25% spell speed because it's very easy to find windows to fit in extra spell casts immediately, but it's almost impossible to track a situation where you had to move right at .5s and still managed to get the slip-cast off due to that extra .05 seconds...but the benefit is still there.