I agree insofar as 'human capital', understood in the conventional sense as the ability to earn labour income, does only a very partial job of explaining inequalities in income.
However, from an investors' point of view, it is a very useful and well-named idea. Human capital is just like financial capital inasmuch as it is a part of our total portfolio of assets. In considering how to invest that portfolio, we must therefore take into account our human capital. Remember that what matters is your total portfolio, not just parts of it.
There are (at least) four general principles that matter here.
First, ask: is my human capital like a share or a bond? If your labour income is relatively stable, it is like a bond. And if you own lots of bonds, you are in a better position to take risk with your financial capital. By contrast, if your income is volatile and cyclical, you might lose on your human capital at the same time that shares fall - which makes the latter even riskier for you. For this reason (other things equal) doctors, who have relatively stable incomes, can safely invest more in equities than, say, architects, whose income is cyclical and so tends to fall in recessions when shares fall.
The second principle is a corollary to this. If you have risky human capital, then you should have a bigger proportion of bonds and cash in your financial wealth. However, as you approach retirement your risky human capital represents a smaller proportion of your total (that is, human and financial) portfolio. With the riskiness of your overall portfolio declining you can therefore afford to take more risk and so hold more equities. The advice that older people should own fewer equities and more bonds is therefore wrong for some people. (In fact, it's wrong for quite a lot of people - but that's another story).
Thirdly, your human capital can give you flexibility. If you have a reasonably secure job which you enjoy, then you can reasonably postpone your retirement - in effect, getting an extra year or two of returns on your human capital. This makes equities a safer investment for you, because if they fall you can top up your wealth by working longer. If, however, you hate your job or have an insecure one, you don't have this luxury.
Fourthly, younger people face especial risks to their human capital. Returns on it might be depressed by long-term secular stagnation or if robots replace human workers. One way to diversify these risks is to invest overseas. The case for investing in emerging markets equities is not that they offer 'growth' - which has always been a silly idea - but rather that they offer a chance of diversifying out of some of the long-term risks facing western economies.
So, while human capital might be an ill-named idea for the purposes for which it was originally intended, it is a very helpful idea in a different context.
