The most infuriating part of the app store, and there are many, is the fact that Apple has given developers no way to give a sample of a product with the ability to upgrade to the full version within the app. What this has created is an environment where each paid app has a "lite" version for free. In theory this is a working scenario, except for the fact the lite version has no direct connection to the full version.
Apple has, finally, given developers the ability to write one application that has a free demo mode and, if you like the application and want the full version, you can purchase it right through the app itself, without having to go back to the app store and purchase a different app and install it, then uninstall the lite version.
Now, for a messenger or something, this might not be the way to go. The last thing a developer of a paid app wants is to release a demo that might have enough features to content people, leaving no reason to upgrade. A game, on the other hand, could offer 2 levels for free, then give you the ability to upgrade to the full version for $9.99 without having to reinstall (as they do on essentially every other platform ever).
I would assume few developers are going to go back and rewrite existing apps to take advantage of this new feature, but most new paid apps will.