The point is not that we want a DK to hit a yellow attack then sit around waiting until they can do something fun again.

The point is that we have no room to add anything to the DK rotation. If we wanted to add "clearcasting" style procs or cooldowns finishing sooner or runes regenerating sooner, we can't really do that because the DK doesn't have room in a very tight rotation to use those abilities. If you try and use your GCD on anything else, then the whole rotation collapses because then your runes aren't coming back at the right time or your diseases are expiring or whatever.

Imagine 1-4 are rotational abilities like Icy Touch and Blood Strike and F = "fun thing".

DK rotation today: 1233331233334123333123333
DK rotation in Cataclysm: 123333-F-12-F-33334123333-F-