Really? Evil? I dunno -- I would definitely agree with you if it was an audio slide show or movie that just started blaring at you. But the usage I'm thinking of is more like a portfolio page where you want images to change automatically. Or you can use it to display images and messaging. I think a fair number of sites are using auto slideshows on their top pages.
You can see a couple different ways I have used autoplay slideshows here:
http://albanyheadshots.com
http://upstatephotographers.com
In both of these cases, the coding a user has to do, while not difficult, is tedious and slow -- nowhere near as easy and elegant as Zenfolio's new embedded slideshow.
Zenfolio's slideshow implementation is superb -- I spent a few hours playing with it this morning, and it is flat out the best I have seen anywhere on the internet -- except for the lack of autostart, and no audio. My guess is that if and when you implement these features, you will be shocked at how much usage the embedded slideshow option will get.
All the best,
Paul