And here's an email I just received from my aunt (I had sent the article to her):

"A combination of people being unaware of it and marketing.   Sonoma, where this article takes place, used to be a low-cost alternative to the Napa Valley and Napa, so properties weren't marketed to their income range.  Like all properties in No. California, home values have shot up so now shared housing has moved into that area.   Living in Tiburon and Belvedere [REALLY ritzy neighborhood in Marin Co. directly across the Bay from San Francisco], we received marketing materials on these offerings regularly.    The Zillow people are new to this concept and they have the ability to really market!"

As someone who has many times done the whole VRBO route (AirBNB), I think it's much ado about nothing.  People who pay $600,000 for a PIECE of a property are NOT going to be rabid party animals.  Renting a mobile home in Las Vegas for $100 a night?  Yeah, it's gonna be trashed.  $600,000 a year?  Um, no.  And I disagree that these people are trying to get around the whole timeshare thing.  These are NOT timeshares.  These people are home owners and they want their property value to go UP -- timeshares make as much sense as buying bottled water as far as I'm concerned.  If I had the chance to buy half or one third ownership of a moderate house near Disneyworld, I'd go for it in a heartbeat and would certainly take care of the investment...and NOT bother the neighbors.