He's come off the bench for four out of his five seasons, either on losing teams or ones hovering around .500, of course, it stands to reason there's not going to be the continuity (minutes, lineups, and personnel) to where he's going to come out having a positive plus-minus. Once again, showing your inability of being able to contextualize stats properly.
It says less about him and more about the situations he was in.
If he's in the starting lineup for the Warriors and they stay relatively healthy, he'll come out with a positive plus-minus at the end of the season. But what does that prove? It's a meaningless stat to hang an argument on in this context. His plus-minus shouldn't be the center of discussion - it should be about does his play lead to winning basketball.
Oubre will provide them with more impact than a pure shooter will. His versatility, defense, athleticism and energy will be of more benefit than someone who's primarily a pure shooter.