Brad, note that Gary concludes that TVPI will show a higher value when the return on the non-called money is added to total return. He doesn't say that it is a good thing.
Gary is correct that this return should be added, but the difficulty with capital calls is that an LP is usually only asked on short notice to provide the capital. Therefore the LP will probably place part of his commitment on a short, low yielding, investment. A side effect of this increasing use of credit lines might be that LP's will increase their commitment above the amount they actually want to invest as part of it will be financed by credit anyway.