Earlier quoted context omitted.
And as a site owner, it has the bonus of being much easier to track than RSS. It's difficult to quantify how many people "read" your RSS feed in a week. But it's pretty easy to count how many people loaded the images in your email or clicked a link.
Dosn't email clients such as gmail automatically block images though? How do you track it?
Yeah, but (and I'm only one data point) I always click the "Always display images from .." link whenever I open a legitimate e-mail that looks like it's missing images, so they get to track me and I get to see a complete mail ;-)