I don't worry about one of those brushes holding onto dirt and not rinsing clean, at least not one I'd maintain. I have no doubt they release dirt. I have absolute faith in the above linked review. The issue I have is feel. Whether you use a brush with a pole or without a pole, you just can't feel the paint like I like to feel the paint...rather feel for debris...during a maintenance wash, which to me is supposed to be a non-aggressive approach to washing, like I can feel with a mitt. If I feel something out of place I STOP. Fingers are one of the most sensitive parts of our bodies. A pole, and/or bristle's take that feel away to a great degree.
Again, using the coin-op or home pressure washer, before using a brush or a mitt on the paint, negates most of the worry of dragging something stuck to the paint across the paint and scratching it, as does weekly/bi-weekly washings. Tree sap, bird droppings, airborne particles particularly around harvest time can float for hundreds of miles...they too can present scratching issues and become stuck to the paint from morning dew and warm sunny mornings so blast them off first regardless of your choice of wash tool.