Uma that is a really good question, it is the point I keep trying to make about unschooling, but I can never seem to put it the right way! :-\
As far as I'm concerned, it was never my decision to be HE'd or unschooled, it was a choice made by my parents. Although I can see the good points of HE, this is my ultimate problem with it. Many people would say it doesn't matter, because you can train for a particular career later on, BUT adults have to pay for their education. Plus what do you do in the interim years, when you could be embarking on a career that you love? Don't want to bang on about my own situation too much, but I have ended up doing a variety of dull and low-paid jobs while I study for an Open Uni degree (which is costing me twice - both upfront in fees, and because I am not qualified to earn a decent wage until I have completed my studies.)
However, I have 2 sisters and a brother, also HE, who have been very successful academically and career-wise. The difference between them and me tho, is my parents arranged for more formal schooling for them as soon as they reached teenage years. One sister works in IT, the other works for a global engineering company, and my brother is at Uni studying Avionics. So you can have it both ways, but you need to ensure that your kids get the right bits of paper, and more importantly, the right kind of study skills and confidence to pass A levels or equivalent, so they can get into uni (and succeed there.) Basically there comes a time when you have to stop 'unschooling' them and start 'school at home'. Otherwise, yes you are going to be limiting their path in life. Personally I don't think parents have a right to do this; I don't want my son to be limited when it comes to future career and study choices. (If he chooses to become a shelf stacker in Tesco's, well that will be his choice, and it will be a free choice IYSWIM. As long as he is happy!)
Uma, your approach of having 'unschooling' at home, as well as your kids going to school, is just what we are doing with DS and it works really well for us.