I don't see the reason to wager the price of a couple of beers to see if my knot will hold on a fish that has taken me down to the backing. I fill the spool with pp, and then maybe switch ends after a coupla years. This works out to be the same cost as using filler, with no knots and good stuff right down to the tape covered spool.
If you want to match your 20 lb pp to an equiv diameter mono, that means that you will be filling your spool with 5 or 6 lb test. Is that really the situation you want?
I understand that if you are jigging walleye, or casting for bass, you might typically use less line, but for the downrigging, trolling, flatlining suff I do, there is no backing on my reels.