We had exactly this at Bloc (formerly Trybloc.com).

There's definitely space for this, but what we've discovered is that it has the same problems as Codecademy-esque services. You can do various things to attempt to keep users engaged, but at the end of the day nobody is paying for anything and therefore has very little invested in the process.

That's not to say it can't work -- perhaps it can. But it's been tried and the variables need to be tweaked just right, and it's a hard combination to find.

I agree, I'd love this too. I learned in a similar manner as what you've described.