I'd like to posit that the reason you've seen more of the latter is because you've hired for more of the latter. I assure you, if you were to look for well-rounded, motivated developers that didn't have the immediate skills you need, you'd get the same work done. Usually more effectively, since the people who don't have the skills usually feel they have something to prove. Their work turns out better.
Unfortunately, there's not really any way to prove this. I wish there were, since it'd be quite nice to demonstrate it. All I can do is say what I've seen.
I don't think that I'm inherently biased against promising candidates with no skills. Heck, I used to be one myself. Perhaps it's just that in the fields and company types I've worked at most (startups) that's not usually what you look for. The promising recent-graduate scenario is much more common in big corporations where people plan for long, linear careers. In the startup world the average time for someone to be at the company is 2~3 years, and the roles usually get filled only when there's a burning need to get the job done right now. So it could be that I'm not aware of what the correct proportions between the different tracks are.
Unfortunately, there's not really any way to prove this. I wish there were, since it'd be quite nice to demonstrate it. All I can do is say what I've seen.