cavitation is bad.
it is actually boiling of the water due to low pressure on the following side of the blade. and the subsequent crushing of that bubble from steam back into water from the blade moving towards it causing a high pressure front.
This can damage the blade face eventually eroding the blade face.

the plate is supposed to help eliminate this by limiting the amount of pressure drop as the blade moves away from an area. Because the volume is more fixed in that space, it cannot drop as far as if it were wide open.

If I were to guess, you would use less power moving due to cavitation rather than more. Since vapor is compressible it would not require much effort to move into that space.

the 8 blade wp, spun slower, for non-a/c cars.

6 vane spun faster for a/c cars.