Free apps are a trend right now but in one case or another a mobile app consumer will want to try a paid-app. These apps usually has the features or value they need (which rivals free/limited apps) but is limited by the need to purchase. This innovative idea will enable these consumers experience paid mobile apps in its full version, not 'lite' or 'freemium'.
Examples of who offers to try apps in an innovative way is APP.IO, APPSURFER.com. The plus for this idea is that it is not limited to be tried only as an ad(try for a couple of seconds) or needs a browser to try.
Will this be legal? Yes, as the developers themselves will submit these apps with details as consumers see in app stores. A platform/system/technology/API will be built to cater this to consumers to add more value-added experience to these apps (ie. browse apps and try, no need to download or install). Developers on the other hand will also benefit from the idea where they get leverage in return.
There is more to come for apps, and it's not over for paid-apps that can give far better value and experience.