Believe me, I have a lot of sympathy for this viewpoint. We don't want people continuing to use IE, so it seems logical that we should make their experience "lesser", using CSS3 for browsers that support it and falling back to flat square boxes in IE, and that will force them to change browsers, right?

Unfortunately, my experience over the years has brought me to the conslusion that this assumption is incorrect. If an IE user comes to your site and it doesn't look pretty, they're less likely to switch browsers than they are to just go to your competitor's pretty site instead. It sucks, but that's the reality, and that's why almost nobody gives IE a totally degraded experience.

Instead, web developers use all sorts of tricks of the trade (sliding doors, complex overlapping sprites, empty divs positioned to corners, etc.) to "fake" the features that CSS3 gives us natively, solely because they are forced to support IE. That's what PIE was created to change. It's not about giving IE users a better experience (most sites already do a pretty good job of that already), it's about making the web developer's life easier so they can deliver that same experience with less code and fewer images.