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Also Theoddmanout.net to go to a demo for 1000 Amps.
Adding a demo can actually reduce the number of people who purchase the game overall. Best case scenario, both the game and the demo for it are awesome. Even in that case, folks will get turned off as they just find it isn't awesome for their own enjoyment. If the demo is meh or bad and the game is great, the game gets screwed. If the demo is awesome but the rest of the game is lackluster, the player doesn't recommend it to other folks after buying in because they ended up disappointed. If both are bad the devs get cut off from the pittance they would have made out of ignorance.
It just doesn't make much sense from the dev's perspective. Besides, at $5 for an indie game, the barrier for entry is already quite low, and there are always bundles. About the only place it really makes sense right now is mobile, where there is tons of free-ish competition, and little or no publicity, such that it becomes a necessity to compete.