The idea behind not sharing the idea up front is simple. I have plenty of feedback that the idea is pretty good. I don't need further validation, at least until I have something to demonstrate. What I need is people that I am going to love working with, and who have the best chances of turning this into something really cool. I figure the best way to do that is not try to win them over with the idea, but with the type of team I want to create. The last thing I need is people liking the idea and then consciously or subconsciously overlooking, or even hiding, the differences in our style/values/goals/personality in order to work on the idea. No idea will be strong enough to hold a team together if we are simply too different in all the other areas.
I may be wrong, but this is how I want to build the team. I have had enough people contact me to express interest, as well as others who have emailed to say they wished they could code just so they could join me. And those that I have met in person have all agreed that this strategy is likely to lead to building a very strong team. So at least for some people this approach seems to have some appeal. But I understand it wouldn't work for everyone, and I'm fine with that.