Let's take as an example my favorite team, the New Jersey Devils. They are having quite a bizarre season so far - 8-1 in shootouts, their record in regulation is a mere 10-15, but right now they're holding on to a playoff spot. More strange is their special teams play - they've scored a mere 17 power play goals, while allowing a whopping total of 11 short handed goals. Yet while shorthanded, they've only allowed 10 goals and have scored 7 short handed goals themselves. In addition, they've allowed 4 empty net goals and have scored none at even strength. All this has made the team plus minus even more useless than it already is, since as we know, plus/minus includes short handed and empty net goals.
Let's just look at the top 9 forwards' plus minus:
Looks pretty bad, right? But when we take out short handed goals for and against as well as empty net goals, it looks a lot different:
|Forward||+/-||+/- at ES||Difference|
We see a lot of pluses where there were minuses before. If we were using +/- to talk about even strength play on the Devils, most players have a radically different even strength +/-. We might erroneously think that Ilya Kovalchuk is having a horrible season at even strength, but he's merely been average-ish. What are we even looking to describe when we talk about +/-?
Conclusion: Plus/Minus is stupid. Again.