You're not understanding the hive mind. When the buzz starts building, they wont need to play it. You're right about it's good to give demos, but limiting the demo to 50 percent of the population is even better. Why do you think they start by releasing it to a VERY limited crowd at game conventions? Also, not releasing info on the 360 exclusive content builds even more interest.
The big problem is after global ec. problems people are more paranoid on their buck. What was acceptable with say SC (guest characters in SC2) is not acceptable by todays standards. Why do you think certain games delay when they release in the same month as another game of the same genre/etc.? Because of competition. If we say had one fighting game coming out this year or around the last few years, I could easily agree.
However, on another note, if the content was crap a wise consumer who may look at the game and go "wow, I'm getting ripped off of this experience, forget NRS and WB" because they may only a 360. Which would deprive a sale. That's just one, however, when discussin a group we don't have just one person, we have multiple sales.
Sure the standard "I'll buy anything with hype including Halo series" person [Note: not hating on Halo I like the series just making my stereotype more apparant] will buy it, but people are way more conscious on their purcahses now. Which again in the long run lowers the potential amount of sales. Where as if they showed the content/etc. right from the bat, it gives more incentive to buy right off the bat. Because the smart consumer won't buy if they hate the extra content anyway. [Which also stops them from buying another potentially buzzed game.]
In a year with GoW3, Disgaea 4, and more, you really have to put your hype, where your mouth is.