I'd imagine that if the Demo contained all the game files, someone would have cracked it into the full game by now. I think it's safe to assume that it's just a demo.

But either way, there's no harm in leaving it until you get the full one.

I could be wrong, but to my knowledge the demo is useless once you own the full game.

Before I got L4D, I still had L4D Demo in my account, and it said "Pre-load complete", and asked me to Purchase the full game in order to play. After I got L4D, it showed a separate game from the demo in my games list, and installed separately from L4D Demo. I didn't delete the Demo until after finishing installing the full game, but once I did delete it, it had no effect on the full game.

So I'm pretty sure you can remove local content of L4D Demo without having to worry about it messing up L4D.

Edit: Another thing is when I downloaded L4D, it started from 1% and went on up. Whereas usually, when you have a game pre-loaded, installing the full version of it begin the update at like 60% or so.

So the L4D demo is just a demo, and it not connected to the full game as far as pre-loading or downloading content goes.

Yes, demo is 100% useless. I left the demo installed whilst downloading the official L4D files, and it was still 4gb, regardless. So yeh, no point on keeping them files.