The one thing that always gets my goat - as an ex lecturer - is that hardly any employees looks as your college marks after the first job. It is all about "who you worked for" - which says pretty much nothing (because it would be "who you got to give you a job", not about how good you are).
Just because someone has got a drivers licence doesn't mean they can drive an F1 or Champ car - why does anyone think that someone who who has a college degree is going to be at home with cutting code in a distributed HA environment.
I can tell you, without exception, everybody that came out of our university course with marks of 80/100 or above was smokin hot. If you looked at those marks even 10 years into their career you would see they are still a reflection of how good they are. Google's grabbing of PhD's is similar to looking at the marks and picking the top students. If you pick someone who has scraped though, you're probably going to get a lemon, but if you take the pick of the litter - you get stars without having to groom them yourself.
It takes 10000 hours to be an expert at anything. Your contact hours in college may be as little as 2500... If you just give people on the job training - it costs your business lost productivity while the mentor helps out for, lets say, the first 2500 hours, so 62 weeks of Full Time Equivalent work for a grad... lets say that comes out to $50k to make it easy. Now lets just say we have about 28k of lost productivity from the mentor. That is a $78k undergrad degree that the business has just funded (and the employee can walk away with that). Now you can top that with things like 'health plans', 'insurance', and 'opportunity cost'. Opportunity cost is the big thing, because that $78k might have been a few fully competent employee which could bring the business in more money. .. hence the existence or tertiary education....
Depending on how much your mentor is paid, you are probably looking at an expense similar to the cost of a college degree (if you add equipment, taxes, opportunity cost, two sets of wages etc). It just that you are getting business to wear the expense - and then the employee walks away with the benefit. Great if you are the employee.... crap if you are the business!