A lot of XenForo customers are not developers or designers and would greatly benefit from a simple import/export template modification process. For example, want the "Stay logged in" checkbox checked by default? Import this template modification XML file.
I can see a lot of great tweaks and mods for templates being shared this way without the added overhead of creating an add-on. The key would be to keep it as simple as possible focusing on single template modifications .. not styles, packages, addons, etc. If someone needs to share or distribute multiple template modifications then they can package them together in an addon.