I’d like to add one thing I have been thinking of:

there is one condition under which empty results of tabble combinations could be useful. That is, if you could save tabble combinations.
In Evernote, you can tag notes and combine these tags with search terms (sounds familiar…? 😉 ) and then you can save these searches. For example, I stored a search for notes with unchecked to-do items and the tag "Blogging".

The result list of saved searches always varies and an empty result can be a good result (especially with to-do lists… 😆 )

Saved searches in Tabbles (I haven’t found them, do they exist?) could be really useful. The result list for a tabble with an auto-tag rule does change when a file is added, e.g., but an automatically changing result list of a tabble combination (say, "marketing", "videos" + search term "uncompressed" in the file name) would be great.

Just a thought.