totally disagree with that philosophy
if a buyer visits a site, they should be looking at the domain names, not trying to figure how much money the site owner is or is not making.
adsense can provide income to pay renewal fee's for not only the domain that the portfolio site is attached to, but also provide income to sustain the portfolio or a portion of it.
and if the names are listed on other venues and have sold there as well, then that point becomes less, and less important with each sale.
the site doesn't have to look high end, the names just have to appeal to the visitor.
are you gonna buy domains from them just because the website looks good and has no adsense, but all the names are crappy?
or are you gonna pass on buying some good domain names, because there is adsense or the design ain't so hot?
can you judge a e-book by it's e-reader?