If you ask me, demos are when you want to give a game a try, to see at least a little of how it'd play. They are not the best way to judge a game, but it's a decent little window into the gameplay and visuals at least.
Game reviews on the other hand are when you want to read what someone thinks of the game.
I generally don't base my purchases on reviews or scores, but it's always interesting to see what certain media have to say on something I'm interested in or have already bought.
I do agree that a good 90% of game reviews out there are completely useless, but not because of playable demos. Most magazines and websites simply have nothing interesting to say. Most people want useless facts and a brief synopsis of what's superficially good about the game, and if anything, that's killing game reviews.
You don't read (good) movie reviews for a plot synopsis, followed by "the acting is subpar, the special effects are pretty decent and some of the plot twists are usually predictable" and a score. Yet this is the format and focus of most game reviews out there, and it's no wonder they're pointless.
When game reviews go the way of Sight & Sound (like the UK magazine EDGE), writing reviews and articles that are interesting and entertaining to read even if you don't agree with the reviewer or even care for the game in question. Text that goes beyond only the superficial elements and talks about what actually matters, or at least what should matter if the industry is ever going to hope to mature. That's what I like to read. But then, 90% of gamers apparently don't, so no wonder we can't have any nice things
How ya doing, buddy?