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.
As mentioned, it's not suitable for large audiences - but it's perfect for other stuff.
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.
And yes, it's absolutely fucking awesome. Better than any book reader, online or off, anywhere.