The primary bonus of dynamic haste effects is that of increased focus generation - both through the passive regeneration, as well as faster Cobra casts. Both RF and Blust allow you to GCD cap your Cobra casts if you have T11 4pc, and come close to it anyway without the 4pc. The faster your cobras cast, the more you can fit in between Explosive Shots. More cobras every 6 sec = more damage, as well as more focus for instants, meaning more AS as well. Bumping your autoshot faster by stacking both is not worth negating additional time with faster Cobra casts.