Trial feature should be built-in.
The trial feature should be built-in with automatic enforcement, user prompting, disabling, etc, just as it is in the Windows Store.
In the Windows store, I can choose trial mode for my app, and then pick the number of days I want the trial to last. From there, everything is taken care of for me. If the user uninstalls during a trial, it tracks this fact and prevents them from starting a new trial later.
Contrast this with the Windows Phone world, where the developer indicates trial mode in the store, but has to manually track the number of days since install, whether the user has purchased, etc. If the user uninstalls and installs again later, the developer has no idea. Adding trial-mode in Windows Phone feels like implement-and-pray.
I absolutely love the Windows Store implementation. With the two stores combining, can we bring built-in trial mode support to Windows Phone?