So John McCain doesn't know how many houses he owns. Yep, that highlights how much richer he is than the average American. I don't think the information is very relevant to his candidacy, however. I'd be happier if he were a self-made millionaire from private industry than if he were a pauper, but I guess he's somewhere in between those two extremes... like most of us.
I suppose it would be similar to if I were vacationing in Mexico and met a local who asked how many computers I own. Who knows? I'm so rich, I've got old unused computers gathering dust in my closet! Compared to most of the world, the average American has vast wealth. In general, that's speaks well of us and isn't something to be ashamed about.
The implication behind the question is more shameful: that we average Americans should be resentful of McCain's wealth and should punish him for it by voting for Barack Obama.