Why do people feel the need to rate a game on it's demo?

It's not right, you don't get to feel the full game in it's full flow, i was abit unsure of red faction untill i played the very very breif demo, people should wait till they play the full game before they start raiting the game as poor and alike. It really gets on my nerves when i am looking for good honest sources of information on games to be slamed by kids rating the game down because they: A) Can't play it B) Can't understand it or C) are just plain idiots.

People please refain from rating on demos it's pointless and it's not big, wait untill the full game comes out and give a full view not just "I played the demo and it sucked" that's because they don't want to give too much away, fools.

B.D.