Do you use Windows?
Be rational. Most users use IE on Windows. Even if you don't like it, that's the way it is. Adobe Reader 9 loading in IE can crash the browser. Don't believe me? Google "IE loading PDFs" and you'll see the huge number of users who have problems.
Flash is reasonably reliable in IE. Additionally, Scribd has the option of embedding ads in the PDF more easily by using the player, which gives them a path toward monetization.
When creating a business, you have to be realistic. This is not a techno-utopia where the best technology always wins. In the browser wars, IE is still the current leader. You have to live with it if you want to maximize profit.
Personally I think scribd solves some problem for publishers, while actually degrading the experience for a significant minority of the viewers. Their relationship with publishers have significant negative externalities, and this makes scribd at least somewhat evil.
Adobe Reader used to be really slow. But, even on my almost-three-year-old laptop, Adobe Reader 9 if quick.
I honestly don't know why Scribd doesn't do something to improve their product when obviously so many people hate it. Scribd is one of the very, very few things about my computer that still frustrates me.
Also I'm not sure what metric of 'quick' you use. The iPaper swf is around 150 kb, and the Adobe Reader binary is around 30 MB. Take a look at http://www.dearadobe.com/top_rated.php - most of the top five are related to Reader bloat.
