I kind of agree with you there, its a nice idea in theory, but in reality it just runs out of these grooves anyway, I supposes it'll hold a little more water for a few seconds.
I've certainly never noticed the remotest bit of difference myself.
If you are doing the ouside of large plate glass then you want your applicator to hold as much water as possible, so a big, fat, fluffy sleeve is ideal.
There are other times when you wring out the sleeve because you don't want to be mopping up the water that sloshes all over the place
Ian