That's a good question MobileJay! Actually, that is the philosophy behind one of Gritguards competitors called "Dirtguard". When I went to SEMA this year I saw the Dirtguard booth and talked to the gentleman there who I believe was the owner of the company. "Dirtguard" uses a very porous foam disc, almost like a bug scrubbing sponge, underneath their "guard". The thought process is that the debris will be trapped in the foam when you take your mitt in and out of the bucket. The foam is supposed to overcome the wicking or hydraulic action that is created when dipping the mitt in the bucket.
My humble opinion is that both makers have good points and bad, Gritguard has a large "grid" area for debris to fall into as opposed to the Dirtguard which is slanted and has numerous round holes for the dirt to fall through. I am not sure if it captures as much "stuff" . The flip side of that is the screen in the Dirtguard "seems" like it would retain more stuff and keep it captured or encapsulated.
I think Richard Lin, Octaneguy, has used both so perhaps he'll chime in on his experience with the two.