Opponent Counterpart 48-Minute Production
FGA eFG% FTA iFG Reb Ast T/O Blk PF Pts PER*
19.0 0.485 4.7 21% 5.0 8.9 2.9 0.3 3.7 22.5 18.2
Also, here are the Opp. PER numbers for Rajon Rondo:
FGA eFG% FTA iFG Reb Ast T/O Blk PF Pts PER*
15.5 0.474 4.5 28% 4.0 7.9 3.7 0.3 4.0 18.7 16.5
And for Chris Paul:
FGA eFG% FTA iFG Reb Ast T/O Blk PF Pts PER*
15.3 0.471 4.0 25% 4.9 7.3 2.9 0.2 4.6 17.6 17.2
While Nash is behind both players, his Opp. PER isn't that far behind either player, nor are his points allowed per 48 minutes or eFG% allowed. While Nash isn't good defensively, it's not like he gets torched on a nightly basis; if he did, I would imagine that it would reflect more in the statistics.
You seem to be stuck on PER and yet that is the last of the stats I would be looking at because, as you know, I am not the largest Hollinger stat fan in the world. But Roy, Nash was giving up and average of over 22 points per 48 minutes. That's a huge number. He was being out rebounded and had a 1.75/1 turnover/turnover against ratio. He also gave up nearly 9 assists per 40 and giving up 19 FGA per 48. All these numbers when taken into a basketball interpretation point to a really bad PG who teams went after because he couldn't force turnovers and because he was turnover prone and because they could get shots off against him and given the amount of points scored were fairly successful at it.
He scored and was responsible through assists for a large portion of his team's offense but he personally gave up as much as he created and I would counter that it was because of his team mates that his defensive numbers when he came off the floor weren't a lot worse. Shaq and Amare and Hill and Barnes are decent defensive players that probably hid a lot of his deficiencies.
And if you are going to look at Rondo's and Paul's competition at least compare it to what they also did:
Rondo's 48-Minute Production by Position
FGA eFG% FTA iFG Reb Ast T/O Blk PF Pts PER*
13.8 0.514 4.9 56% 7.6 12.0 3.8 0.2 3.5 17.3 21.0
Rondo's Opponent Counterpart 48-Minute Production
FGA eFG% FTA iFG Reb Ast T/O Blk PF Pts PER*
15.5 0.474 4.5 28% 4.0 7.9 3.7 0.3 4.0 18.7 16.5
Paul's 48-Minute Production by Position
FGA eFG% FTA iFG Reb Ast T/O Blk PF Pts
20.1 0.528 8.4 26% 6.9 13.7 3.7 0.2 3.4 28.5 33.5
Paul's Opponent Counterpart 48-Minute Production
FGA eFG% FTA iFG Reb Ast T/O Blk PF Pts PER*
15.3 0.471 4.0 25% 4.9 7.3 2.9 0.2 4.6 17.6 17.2
As can be seen, Paul dominated his opponent being his team's focal point offensively, much like Nash. Paul outscored his opponent by over 16 points per 48 minutes and out rebounded his man and out assisted his man by nearly double. Also given that Paul's opponents shot nearly 25% less shots than Nash's shows that team didn't go at Paul.
As for Rondo, he didn't become the focal point or higher scoring option for his team until nearly March when it became clear that KG wasn't coming back. Even so it can be clearly seen that teams did not like to go at him defensively through his FGA against and because he turned over his opponent a lot more than than Nash. If Rondo was better than a 4th scoring option for most of the year my guess is his dominance would have shown through as well.
Okay, maybe just average is too strong a word to describe Nash. He is definitely in the upper third of the starting PG's in the league because the system he is in and his great shooting and passing are such a lift. But numbers show he's not a good defender and observation shows he's even worse than the numbers show. he is still a very good PG but people give him a bit of a ride because of his offense and flashiness and always seem to disregard his porous defense.