I've never had much time for that (and still don't). It seems nothing more than a kind of branding exercise. "You'll always be able to identify a book by X because of his/her voice", just as you can identify any other brand--baked beans or breakfast cereal.
Not that I think branding of writers is a bad thing, I just don't think that it should dominate the actual writing.
Despite that, I do think voice is important. But I think that a voice should be the voice for a novel or a character, not for a writer. It's not the writer's voice, it's the character's voice.
There are writers like Bradley Denton and Sean Stewart who seem to be able to write brilliantly in a wide variety of voices. I've always admired that enormously. I'd be hard-pressed to identify a "voice" for either of them, but within the context of a book, they are clear and compelling.
I'm saying all this because I've spent the last few days trying hard to get the voice for the protagonist of the new novel I'm starting, and I'm finding it hard to get exactly right. You see, this character, and her story, will have quite a different voice to the last novel I wrote, and that had a different voice to the one before.
Getting the voice right when you start is really important, I think. It's something that for me is difficult to fix or change in revision. It's about how the character sees the world and describes it. Get it wrong, or inconsistent, and you almost have to start all over again.
And, yes, I know that some people do just that in revision: start from scratch and rewrite. Those people are braver than me.
So, how do you come up with the voice for your protagonist(s)? What kind of exercises do you do to develop it? Or do you, like I sometimes have, just dive in and see what emerges?