Gears of War lead designer "CliffyB" a while ago defended his decision not to release a demo for Gears 2 by telling GamePro:
A beta is like hooking up with a girl just to say, "yeah, I f*cked her." I know that sounds crude, but it's the honest-to-God truth. Once you play a beta, you can check it off your list -- you can say, "yeah, I played it." Then you might not feel motivated to get that initial cherry popping from the proper, final game.
Honestly, we could make a demo, but Gears of War 2 wouldn't be out until February of 2009 [if we did]. That stuff takes times to put together.
Certainly not unexpected coming from this overgrown fratboy.
Hey, "Cliffy", here's another "honest-to-God" perspective for you: demos are like the first date, the time where you meet somebody, get to know them, and decide if it's the kind of person you want to bring home with you and spend some quality time with.
Not everyone's first date is the same as "going all the way", and there's still plenty of magic to be had when you get the full game home, unwrap the disc (with any luck, not slicing your hand in the process of getting that vacuum-wrapped cellophane off), and play the full campaign or all the multiplayer modes for the first time.
Left 4 Dead gave me a demo. I didn't think I would like it, since I normally don't fall for that genre. I played the demo one night. Just one. I now own the game and am loving it. Mirror's Edge gave me a demo. It's just a short training mission. I played it over and over again until I finally bought it for myself for Christmas.
See how that works?
So go ahead and disparage the concept of a demo. Go ahead and whine about how it would be "too hard" or take "too long" to make one. It'll just be $60 I won't be spending — to extend the metaphor — to bring a girl home to bed without having even met her just because you say she's a good lay.
See, that would be "cheap sex".