That's more than you would think. Recalling, I bought World of Goo and Overlord on grounds of the demo (they actually made we WANT to have it in the first place). I was pleasantly surprised especially with the latter, because when I played the full version, I realized the demo had given me opportunities you actually don't have that early in the game, so that was really well done. Plus it came with the opening "cut scene" AND a tutorial. That's a lot for no bucks, other than that it made me pay twenty to get it.
Other than that, you're dead on, I've long lost count on the demos I played only to find out that they sucked and can't be trusted. Which is, essentially, why I pirate - DISCLAIMER: I buy the games if I like them, I relly do. Even if it means getting a worse version of the same, like with Mass Effect, which I bought after beating my pirated version.