It's sort of a lose-lose situation. You want to have stories that reflect your audience, and perhaps reach out to those that might not normally be your audience. But you don't want to look like you're doing it just for the marketing. But at the heart of it, that's what your doing, using a minority character to see if there's a market for it.
Without all the media hype, this would probably be seens as a cool move...like we don't find out about her sexual preference until it comes up in the story. But since there's been a big news story...and we have to assume the info was given out ahead of time by DC on purpose, in order to generate a little hype and free publicity...it looks like exploitation. Which of course it is.
A good example of having a lesbian character without the hype was Elizabeth Rohm's character on Law & Order. It's a show I don't watch, so I don't know her character's name, but she was also on Angel (as Kate Lockley). On her last episode of Law & Order she asks if she's being fired because she's a lesbian. The show had done nothing to indicate one way or another anything at all about her personal sexual preference. There was no big publicity announcement ahead of time. It was just part of the natural course of things. Not a big deal.
Which is the way it should be. Is anyone really going to follow this new Batwoman just because she's a lesbian? Maybe. Some will undoubtedly avoid her, and maybe all DC books, because of her (sadly). But overall, it's going to be whether she's written as an interesting character doing interesting, entertaining things, that will determine if she stays or goes. Her sexual preference, ethnic background, etc., matter hardly at all if she's bady written.
Put another way, does DC really think that John Stewart brings more black readers into comics? Or Robin brings more 15 year olds? We've been trying to make it so that people don't notice things like the color of someone's skin for decades now. I think many of these attempts end up setting us back a few steps.
It's like that old TV commercial, the one where the kid asks his grampa if the kid is prejudiced. Grampa asks him "who is your friend Timmy?" Kid replies "Timmy's my Jewish friend." Grampa says, "well, then you're prejudiced." John Stewart, to me, is just a Green Lantern. To others, he's the "black" Green Lantern.