As a BM or Survival spec, you probably wouldn't want a Null.
As a Marksman with Chimera shot (which refreshes your Sting each time Chimera shot hits), you do want the Null.

I tried to switch between between Aspects automatically as part of my castsequences.
Basically one of the click macro's started off in Hawk, halfway through the maximum characters moved to Viper and then had comma's to the 255 character limit.
It's easier to just have a toggle macro separate from the dps macro.
True, it's one more thing to watch, but the control is good too.