To probably answer my own question:
It appears DefaultFolderTemplate is only used for 2 things (see this code search):
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When creating a new folder, as a fallback if the parent folder didn’t have their attributes, columns etc. set up, which shouldn’t be the case normally.
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To set a background image for all folders that don’t have one set individually already. This is only seen in “(Mini) Icon mode”.
Doesn’t strike me as particularly useful…