1) Would a developer who is part of a workforce in a company where there are other developers use this? Chances are, probably not - he has coworkers whom he could ask to help him work through a problem. This is more efficient than hiring outside help - his coworkers probably understand the codebase and the problem domain much better than he does.
2) Would a student developer in a school use this for his schoolwork? Chances are, probably not. Schools usually have tutors, and the instructor is usually available. Also, students are usually low on money to pay for services like this.
3) It sounds like you are what I call a "lone-wolf developer". You consult for projects as your main source of income for companies that don't have on-staff technical talent (otherwise you'd ask them), or are a single founder working on your own project (or, if you have cofounders they are completely non-technical). You would definitely use this service. How many people are there like this?
The reason why your post caught my eye is because I often find myself in the "mentor" role that you are describing, and have often wondered that question myself.
Yea I see your points but couldn't the same points be made for elance and yet they exists. I don't know the financials but I am using elance right now because coding this thing takes too long for my taste and I want to move on to the next thing.
With that said, elance may be used by someone who has no development skills whatsoever so it will have a bigger market, no doubt.
But, I can see a service like this to go beyond development, it could be about marketing, strategy, SEO, etc...
PS: Would you be willing to mentor me ;)
What you are actually talking about is something along the lines of what lawyers do: you purchase a set amount of hours upfront (a retainer), and then you reach out to them with specific problems as they arise, until you use up all the time you've purchased. Sure, there might be a market for all kinds of professionals to sell their future services for retainers.
With regards to me mentoring you: sure, e-mail me, we can chat more.