The time limit won't be all that helpful when it's unlikely that you'll finish the first level of most games, let alone find your way around a complex productivity app. However, Microsoft is betting that playable ads are still much better than judging a title based on tiny ad graphics. And importantly, your favorite developers won't have to bend over backwards to implement this -- they just have to enable a "try now" button, and Microsoft does the rest. This isn't going to help much if you do most of your software shopping outside of the Windows Store (hi, Steam fans), but it might get you to check out programs that you would otherwise ignore.