That's certainly one way to look at it if it doesn't support someone's particular player they are rooting for.
The bottom line is there are a great number of games, shifts and minutes played. All the players who may be in contention for a Norris are up against the same obstacles.
There's a large enough sample population over the course of a season that makes +/- a useful indicator. Simply micromanaging it by saying it doesn't take a second to second look at who each player is playing against makes me think your assertion is Nick is playing against the best of the best all the time while the other candidates get all their ice time against hacks. All the Norris candidates, whomever they end up being, have played against tough opposition.
Any attempt to explain away Nick's poor +/- this year b/c it doesn't reflect generously on him is trying to sweep unseemly evidence under the rug. We're not playing well defensively and Nick has been a part of that. Is he the main culprit? No. But it's been endemic of the entire team this year to a level we're not quite used to.
Nick has always played against the best of the best. Hasn't seemed to hamper his +/- in the past like it is this year. So you can't just say its no longer a useful stat b/c it reflects poorly on him now. Maybe you've always felt that way, I don't know. I just happen to disagree with your contention that it's useless.