Plus,
I think Frank had said in one post that if you've got a bunch of junk already in a used filter, you could end up using up some of the ARX just to attack all of that junk in the filter, and that's less ARX to attack the bad stuff you want cleaned out of your engine.
If Frank didn't say it...sorry for the misquote, but somebody said it on BITOG, and it makes sense to me.