cmon watching demo's is boring like shit, I nearly never watch a demo.
-you can't respond (in teamchat) to what's happening
-you can't talk shit to others in spec :P (though I could understand a rule against that, np)
-the result is known, nothing exiting
-no control of the viewpoint or who you are spectating
So you're basicly wasting your time waiting for something you already know, or waiting to see 'that' player play (which you could have speced all the time if you were there). It is a bit like you read a book or watch a movie for the first time and someone already told you how it ends (some books are good enough to read twice, but when you know the result beforehand when you haven't read it yet it sucks).