Profile Update on Duplicate Match Should be Fill

I have a profile form that is used for registration. It collects First Name and Last Name (I'm using Drupal). I also have a stand-alone profile form which JUST collects an email address so that people can get our newsletter. Due to the potential for people who may try to fill out the standalone form while they are or are not logged in to the site, I have checked the "Update Contact on Duplicate" box in both of these profiles.

After running tests, I see this does a full update, overwriting and deleting data that is or isn't a match. It seems to me this should actually be a "Fill" operation, or that one should at least be able to choose to "Fill" instead of the full overwrite "update". This is why:

If a person registers for the site FIRST, then they have given their email address, first name, last name. So then they log in. Then they go to the form and subscribe to our newsletter, just putting in their email address again. Now when I go in to CiviCRM and find them, they are just an email address, and their name is gone.

It obviously works fine the other way around; if a person just puts in their email to our standalone form and then registers for the site, all the data is there. But in case they are already a site user and happen to be logged out when they sign up for the newsletter (we need it to be available for anon users), then i don't want a duplicate contact created.