The problem with Scribd is that it tries to reduce your cognitive load by one click. "Open or save?"
But it replaces that horrible, horrible soul-crushing "Open or save?" dilemma with its own inferior implementation.
As mentioned, it's not suitable for large audiences - but it's perfect for other stuff.
Sharing to a select group of people (e.g. a link in a mailing list, or on a small online community - it's perfectly valid in my opinion.
And yes, it's absolutely fucking awesome. Better than any book reader, online or off, anywhere.
Having a document hosted on your own website together with a javascript viewer reduces the risk for users both in terms of privacy and security, as Acrobat Reader is a seriously problematic piece of code and all the remote code is a serious violation of trust between the user and the owner of the site.