I do find that derailleurs with clutches shift better when the clutch is disengaged, though it may be only a small difference.
Why a clutch? Some rear cassettes have a very wide range, like 10-50. This requires a rear derailleur with a very long arm or cage to accommodate the big difference. When you hit bumps that shake the chain up and down, that long cage increases the moment arm of the torque that the chain applies to the derailleur. Long cages allow more chain motion and slap. Extreme of chain slap can make the chain come off the front chainring. The clutch makes it harder for the derailleur cage to rotate (edit: in one direction, asymmetric), reducing chain slap.
The general rule is engage the clutch when riding on rough terrain.
BTW, my mountain bike from late 2014 with SRAM XX1 has a 1x 11-speed 10-42 rear cassette and the rear derailleur has no clutch. It shifts great and has never had excessive chain slap. It doesn't have or need a chain guide either. My friend's mountain bike from 1-2 years ago is Shimano 1x with a 10-52 rear, a clutch rear derailleur and a chain guide.
