People do all kinds of things to save money and like everything else if it's ok with them then I guess it's right for them and case over.

I know guys who save coffee grinds and use the same ones 2-3 times over thinking it's a waste to use once and throw away. I also know guys who drive with the same oil in their cars for 10,000 miles or more.

As far as the brush thing goes maybe with a synthetic brush it wouldn't harm the bristles but the knot itself I'm not so sure.