For the general day-to-day XenForo admins, they should make any customisations to their style using template edits. They can use the TM system if they wish, some people prefer it, but overall, template edits are quick, easy, and will make upgrading even easier than using the TM system.
In terms of sharing template modifications, sure. Like you've said, you can do this by creating an add-on... but I don't see why that is "extreme overkill".
The very simple steps to share a template modification are:
1) Be in Debug mode. It's good practice to have a development site or a local install for this.
2) Create an add-on in the Admin CP. This simply involves giving your add-on an ID, title and version number.
3) Create your template modification, or edit an existing template modification and allocate it to your add-on.
4) Export your add-on from the add-on list.
I fail to see how it could be any simpler.
The only way it could be simpler is if you didn't have the steps of being in Debug mode or creating an add-on. But they're still steps that only take a couple of seconds to complete.
