A slipper clutch "slips" when you release the clutch in a gear too low for the tire speed at the moment of clutch release. This slipping action prevents the rear wheel from locking up and skidding. The more slip, the sooner the rider can dump the clutch when downshifting.
Watch Stoner or Rossi slow down from 200MPH to 60MPH in preparation for entering a hairpin turn. They use no engine braking. Monobloc disc brakes are much stronger, (and therefore, much quicker), than engine braking and stop the bike much quicker. So instead of downshifting one gear at a time, employing engine braking along the way, (like we do on the street), motogp racers grab as much front brake as they can while simultaneously downshifting from 6th to 1st gear, without stopping at any gears in between.
So, on the track you want as much slip as possible so you can dump the clutch as quickly as possible when downshifting without having to stop at any gears along the way. 6th to 1st baby, with no stops in between! That's how they be in motogp! Additionally, since no engine braking is employed, it's of no consequence.
In contrast, on the street, where we tend to downshift one gear at a time, employing engine braking along the way, we want engine braking and need less slip.