Demos have been around for years. The only thing that's really changed about them is that they're now available digitally, rather than stuffed into some magazine in your mailbox. The greatest thing for both a consumer and publisher is when a demo works as intended: the player plays the demo, absolutely loves it, wants to play more, and decides to buy the game. It can really help players who are on the fence too, about taking the $60 plunge.
But Schell games CEO and industry analyst Jesse Schell has tried to warn developers that releasing a demo before launch can hurt sales.
According to PCGamesN, Schell stated at DICE 2013, “You mean we spent all this money making a demo and getting it out there, and it cut our sales in half? Yes, that’s exactly what happened to you.”
Schell reminded developers that without a demo, gamers would have to buy the game to see if they like it. Demo downloads can deter potential buyers (after all, if you download the demo and didn't care for it, why would you buy it?)
Schell said that the games selling the most were those that were marketed well, not because they had a demo.
The one thing Schell seemed to leave out is the fact that, if a game has a shoddy demo or is indeed a shoddy game (or simply not to one's liking) then it's to the gamer's benefit and the industry as a whole that we don't contribute to buying those games. After all, in my personal opinion, sales should indicate what players want, not what players were tricked into thinking they wanted.