Ellen and Rosie don't have a tenth of the influence that Oprah has. People don't go out and read books by the dozens because Rosie says she liked it. No, people hate Rosie, they make fun of her constantly. She's a joke. (I don't have a problem with her - I'm just saying, in the eyes of the media and most of the population...)
Ellen, there's a lot more respect there. But still, who is listening? All the folks that are already gay, or are already alright with gays. She didn't have a talk show until she came out. She isn't doing anything mindblowing.
I also think it would be a huge advancement simply because she's black.
There is SO MUCH homophobia in black communities. If Oprah couldn't help out with a political advancement, it could at least be a social one.
People listen to Oprah. She'd, at the very least, get a chance to speak her mind.