I, too, sometimes see lots of dirt at the bottom of the bucket without grit guards. Fact is, when you dunk your mitt in there, the dirt isn't just sucked down there, it takes awhile. Regardless, as I've stated, my car stays pretty swirl free without the use of grit guards.
And if the rinse bucket has significant or noticeable dirt in the water.. just safer to start with fresh water instead of relying on the grit guard, which like previously stated, takes time for the dirt to settle down.
I don't know why I'm posting in this thread again, I guess I'm worried some newb is going to read this nonsense and take it as gospel. Yes, wash/rinse water gets dirty even if you have a grit guard...if your car isn't dirty, why are you washing it??
It doesn't take "a while" for the dirt to fall to the bottom, there is a difference between "dirt" which makes the water dirty, and "grit" which is, for example a particle of sand, which is heavier than water and will fall to the bottom. What the grit guard does for you is reduce the chance that water turbulence will stir that back up into the area of the bucket where your wash media is, keeps the wash media from contacting the bottom of the bucket where that layer of "grit" is, and gives you a surface to agitate the media against to release the "grit".
If you guys don't want to use grit guards, fine, don't...but don't start with this magical thinking that gravity doesn't work, that a grit guard is supposed to keep the water from getting dirty, etc. On the flip side, using a grit guard doesn't magically mean your car won't get swirls, or magically protect against poor wash practice, etc.
Ok, is a Grit Guard overpriced at $10? Yeah, probably. But Grit Guards never break (unless you do something colossally stupid), they never wear out, and in comparison to the cost of wash media, a minimal investment...as I said before, cheap insurance.
And because you guys have me going now...if I came on the forum with something like this: "I bought $30 of towels to do the GD (gosh darn) wash method, but sometimes I wash two cars in a row, is it ok if I just rinse off the dirty towels and use them over instead of machine washing them?" you guys would be all over me telling me how cheap I was being and don't take stupid chances with your paint...it's CHEAP INSURANCE to just buy a second set of towels you would say...