I alluded to this in my earlier response. I think it depends on one's definition of an incubator. I consider accelerators to be things like Y-Combinator, 500 Startups, Launchpad.la and some hardware specific ones. There is an existing team, often with a product and even some funding. They apply. They are admitted into a 3month program or so. Surrounded by mentors, but probably more useful as a way to pitch to a lot of VCs that you wouldn't otherwise have seen. All for a little bit of cash for a little bit of equity.
Incubators in my world, are trying to creating companies from nothing. They may have a specific area of expertise or interest. They may not. Generally, they come up with the ideas themselves. Nurture them with time, attention and money. If it begins to grow, they bring in someone to care for it. And then send them off into the world with money and their love. :) In exchange, the entrepreneur gives up more equity than they otherwise would have because the incubator bore the risk in the earliest, riskiest stage.
In some incubators, like Science in L.A., they are more like a movie studio or producers, where they put together teams and attach them to projects.
It's worth thinking about what variation you are. And by the way, I'd never think an incubator or accelerator know more about your business than you do. They help with all of the other stuff. Oh, but they may be able to connect you with somebody who actually does know more about your business than you do. But that's just a bonus.