I can understand the purpose of Bela's character; I do not like the execution of it. For many of the reasons mentioned above by all the other posters. Also, I think she has already been overused in an unrealistic way... the guys are criss-crossing the country, have been doing so all their lives, and suddenly in the past 6 months or so they keeping running across Bela? In the past 12 episodes she's appeared in 5 of them. (3.3: Bad Day at Black Rock, 3.6: Red Sky at Morning, 3.7: Fresh Blood, 3.10: Dream a Little Dream of Me, and 3.12: Jus In Bello). (That's as many episodes as Bobby has appeared in, by the way.) It is perhaps the cilantro effect -- a little would be okay, but too much just ruins everything.
Another issue I didn't see raised was her interaction with Gordon in Fresh Blood. I didn't like it -- Gordon was one seriously badass hunter, who had never needed help tracking down his prey before. I don't believe he would have stood there and bargained with Bela. He would have tortured her for the information, or shot her because of the inconvenience she caused him, (because he paid lip service to saving humans, but seemed willing to lay waste to anything in his way in his quest to kill Sam) but he wouldn't have put up with her taking something valuable from him. That whole scene made Gordon look powerless.
Also, I still don't buy Dean shrugging off Bela shooting Sam. Dean is so rabidly protective of Sam that he would never just let that slide by. He might be willing to overlook the theft, but not the shooting.
Not to mention I'm still waiting to see more than one facial expression, and to find out if she's capable of a vocal intonation of more than smug condescension.