Quote Originally Posted by dbltoe View Post
My reasoning behind the "This method will lose all changes upon cart upgrade." is that they just won't be looked for by the one upgrading. We stress to use the override system which, in itself takes the upgrader away from classic, template_default, and even responsive_classic if they were currently using a clone of one of the others.
Also, I recently installed a mod that had template_default and YOUR_TEMPLATE files with several of the template_default being new files. Not overwrites of core, but new files.
I can see the point in having something to "fall back on for testing" but can't wrap my head around putting a new file anywhere but YOUR_TEMPLATE.
Just like the template_default files provide a fall-back for the Zen Cart base files, if a plugin introduces brand-new template files (take One Page Checkout for example), it can be beneficial to place those new files also into template_default.

That provides a base installation and allows for template-overrides on those files, so a store can make their own modifications to a file and still have the as-shipped version for reference.