That would probably be very annoying as a general case, as it would badly spam re-uploads.

I didn't emphasize it's meant for first upload not for re-uploads.

Closed WONTFIX per comment 1.

Since commons is using javascript to fill it, prefilling the description for first upload is a valid use case.
Commons may not benefit from it due to language issues, but other wikis would be able to.

What would the default text be? Nothing?

I suppose so. MediaWiki:upload-default-description ?

Change 78468 had a related patch set uploaded by Brian Wolff:
Add optional message to define default description for Upload

https://gerrit.wikimedia.org/r/78468

It will now be possible to edit [[MediaWiki:Upload-default-description]] to set the default text after Brian's patch gets deployed. Closing as fixed.

Change 78468 merged by jenkins-bot:
Add optional message to define default description for Upload

https://gerrit.wikimedia.org/r/78468