Put yourself in other's shoes. If you hear for a game and like to play/buy/read about it, would demo be helpful for you?
To me, Demo (limited in levels not in time) is great asset to spread the word of your game and build a fanbase, but is also the best way to get something much more valuable than that. First impression is key of your success. If the first 1-2 minutes prolongs to 2-3 minutes attention, you have a good chance. It can be used to gather data, and player feedback/critics/suggestions.
But the most important is what type of game you make. Not all games are suitable for demo.. Good luck!