It is hardly earth-shattering to expect the same of mobile apps. Palm software was doing this about 10 years ago (and probably others, I just have first-hand experience with Palm). Demos would also save developers from having to give away promo codes to their apps in order to get their apps reviewed, and it would help ease the concerns of those who wonder whether an app received a good review just because the reviewer got a free copy of it.
Demos might not be a big deal for a US$0.99 app, but once you start edging over $5, it starts to become more and more of an issue. By $10, I'm giving it serious thought, and by $20, I'm pretty hesitant. What if something is priced at over $20? Well, let's just say that I've been looking forward to OmniFocus for the iPad for a long time, but I still haven't bought it.
The absence of demos hurts developers, too. The lack of a "try before you buy" is a big justification for piracy. Even if we ignore that issue, though, look at the time and energy that Omni Group has put out in order to try and convince people that OmniGraffle and OmniFocus for the iPad are worth their asking price. Rather than just releasing it and letting people try it for themselves, they've had to do YouTube videos and a whole lot more in an effort to sell you on something that they've built. I'm willing to bet that they'd rather spend that time doing programming or technical support.
I hope that Apple is planning to make it easier for developers to allow for time-limited demos, but I can only assume that, in order to do something like that, developers would have to add code to their applications to enable a demo. We haven't heard about any such functionality in the iOS SDK, and it would be surprising if such a big feature had been kept a secret.
[Currently, the iOS developer agreement forbids time-limited applications for sale in the App Store; apps must be fully-functional. This does not preclude developers from using the in-app purchase mechanism to add features, game levels or extended functionality, which covers some but not all of the potential needs for demo versions. –Ed.]
The only other alternative I can see is Apple enforcing some sort of time-limit by forcing your iOS device to "check-in" periodically (to see if the time has expired). Such a system, which seems completely reasonable to me, would no doubt spawn a wave of privacy induced, fear mongering blog titles like "Apple is watching which apps you use!"
That said, I have to admit that this is Apple, and they might come up with an elegant solution that hasn't occurred to me. I'd love to be surprised, but I'm not getting my hopes up that this new section of the App Store is anything more than another way to organize existing apps.