Actually, the producers have stated numerous time that Rumpel is indeed a central character. In fact, they just said that in a recent interview. They didn’t even mention Regina. I think it’s over in the spoiler section for season 2. Lana even mentioned it.
The main characters are Emma, Snow, Charming, Rumpel and Regina. You don’t get your own promotional poster if you’re not a central character. The writers give Rumpel so many character centric episodes and put him in the middle of so many of the show’s plots for a reason. You don’t see Rumpel as a central character, and yet you complain that he’s in too many of the plots. If he’s in so many of the plots then it’s obvious that he’s a central figure.