Here's a meatspace comparison. You talk to two salespeople. One is plain looking in plain clothes that could be bought at Target (or some equivalent box store). The other has a similar message, but is stunningly beautiful and wearing amazing designer clothes. Do you trust one more than the other (assuming all other factors are equal)? How many people would walk into a car dealership and approach the salesperson who was most attractive?

I think some "pretty" design smells like salesmanship to the savvy consumer, and this is reinforced by the fact that many of the most valuable sites (Craigslist, Amazon, Google, Ebay, Gmail, Del.icio.us, Reddit) aren't pretty to look at.

This isn't about usability and it isn't about simplicity. It's about the spectrum of ugly ranging to beauty and whether people have a bias against pretty/slick design (I think that a lot of people do).