A couple guys had an idea for a cheap computer and a great search engine, respectively. And they executed it well. You're kidding yourself if you think the Steves sat around and said, "what is it we should do with our lives? We need an idea." They knew what they wanted to do, and did it, and continued to believe in it (in Steve Jobs's case, for his entire life).
It was their passion for the ideas that determined how successful their respective companies were, perhaps combined with traits about themselves. Do you think Apple will ever be as successful without Jobs's dedication? Maybe successful, but not innovative, and doomed to eventually run out of steam. Google is already failing because they are losing focus on the idea that made them great.
I don't think a company can be truly successful unless the people founding it truly believe that the idea will change the world (and how can you truly believe in an idea that you haven't discovered yet, and that you have to go to an incubator to work on?). Of course, this is meaningless in today's perceived-value bubble, which might be why Y Combinator is trying it.
In the end, and I hope I'm wrong, I suspect this will draw out people who are just looking for a cash-out vehicle. "I heard there was money to be made in Silicon Valley, but I don't know what company to start. Help me find an idea!"