In short, it's a bad stat and it';s not something you should be basing your opinion of a player on, unless it's a crazy outlier from the rest of the team.
+/- is really only OK when looking at a game-by-game basis. If your team wins 6-5, but you had a player at -5, it pretty safe to assume they had a rough game defensively. Conversely, if you lost a game 6-1 and had a player at +1, it's safe to say that he was probably the only player who had a good game.
Looking at it over the course of the season is not going to tell you much.
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