I tend to agree with you on this point, but how about if the marginal cost of PV roof tiles over normal roof tiles is 0.  That is, what happens if painting or whatever a reasonable efficiency PV costs no more than a similar no functional coating?  This is not completely unlikely, given the amount of research into thin film, organic, self-assembling PV coatings.  If you could get PV paint-in-a-can in a variety of colours with a similar life expectancy to normal paint, and get useful energy out I can see the price of PV electricity being very competitive.