Id say stacking one shot is fine, it allows you to pull the trigger an instant before the ball is there and the ball is still fired.
Stacking several shots can be bad, yeah its open to abuse, but so is almost everything in paintball.
The reason I am against stacking more than one shot is if your loader slows down for some reason (ie a jam or low batteries) then you could store up several shots because you are pulling the trigger faster than the hopper is feeding balls, this may result in your marker continuing to fire after you stop pulling the trigger.
Then there is the time that a stacked shot can live for, most guns will not work in the way described above (stack some shots before the game, then switch on hopper to get the free shots) as the stored shots will be 'forgotten' after a set perdiod of time.
Lastly there is the problem of detecting stored shots. In short without a NPPL robot its impossible. No human can percieve the times we are working with here.
I believe the MS allows 1 shot to be stored for 1/2 a second.
Our board for the angel LCD allows one shot to be stored for 1/4 of a second, in the testing we did any increase on that will not improve the markers ROF but may cause safety issues.