What's nice is that you often don't realize you need a document from several years ago until you've already upgraded through two or three computers, none of which have the (non-app-store) older versions of the software. You thought backing up your data was enough when upgrading computers and installed apps as needed to avoid bloat and clutter.

It doesn't even degrade gracefully. Pages tosses out a lot of formatting and embedded media from earlier documents, which has pushed us to eliminate iWork from our 'productivity' apps.