It doesn't counter it if as is clearly the case for you) you don't understand what that means. Average means average for all players, both starters and reserves. The average starter has a PER well above 15. If you check the numbers for the other starting PGs you'll notice that A) the average PER for a starting PG would be well over 15, so holding them to under 15 would be good, and that B) most of the other PGs have a higer opposing PER than Rondo. Just because you don't see it doesn't mean it's not clear.
Sorry but a 15 PER against just isn't stellar evidence. There are other guys doing that.
Some guys that are under 15 PER - Hinrich, TJ Ford, Billups in Denver, Chalmers in Miami, Ramon Sessions on the Bucks. Need I go on? With only 20 or so games under most of these guys belts though it's someone fluky.
If we look at a larger sample size (last year) Rondo's PER against was 17.6. Like I said Rondo is a fine defender. But placing him on some pedestal is foolish. Both Harris and Rose superior play on the more important (for a PG) offensive side of the ball dramatically outweighs what Rondo is doing.
Harris has a 29.9 PER for ****. There isn't a PG in the world who can make for that kind of disparity with 'defense.' It's not like Harris defense is that bad. It's the same story with Rose.