This is so true, but I think a lot of people haven't had the opportunity to find good role-models in life and something like this could be a first step in the right direction.
The brilliant can be left to blaze their own obscure bildungsroman.
I think the previous comment was talking about the mentors not having time to go through an open-application process, not the people looking for mentors.
Great mentors don't have time, but less-than-great mentors do, and they are good enough for the target audience of Mentii.
The other issue is, great mentors aren't usually plainly available on a website; the very act of finding their contact info/introduction and making a convincing pitch for "I'm worth meeting with" is a great filter for experts who (unlike YC, for example) don't have the bandwidth for an open application process.