I have a node that contains a multi-valued emfield, "myfield". I populate three of the emfields with youtube url's, and save the node. This creates three rows in the content_field_myfield database table, where each row contains the corresponding youtube-url. So far so good.
I then edit the node and delete all three url's, by checking the "delete the video" checkboxes.

The content_field_myfield table still contains three rows, with the youtube-url data replaced with NULL.

I know that when a node is created and the multi-valued emfield is left unpopulated, emfield adds a row to content_field_myfield with the data set to NULL.

After deleting all three url's, I expected to see a single NULL row in content_field_xxx. But instead, emfield leaves all three rows. Is this a bug?

One downside of leaving all three NULL rows in the database table is that the next time the user edits the node, the node-form contains three empty items (along with the "add another item" button).