There is clearly a use case for building a target list for email campaigns as a standalone activity, however for ongoing maintenance I would like to suggest that the most efficient way of achieving this is to add a target list sub panel to the "contact" and "lead" modules.

In normal day to day activity you can then navigate to a contact and update the target lists to which they belong, for example after coming out of a meeting you have updated some contact information and also want to add the person to a regular mailing list.

I did manually edit the contact module code to do this, but reverted it during an upgrade and have not had the chance to put it back into the system.