I think the real question is
Do we need a tag line?
Without looking it up, how many of you know the Drupal tag line?
next question is what does the Drupal tag line mean, and have you ever used it to describe Drupal?
Can any of you name the tag line from an any open source project, without looking it up?
Personally, I don't think we need a tag line, let alone a new tag line. Look at the examples from the new Drupal style guide, and you will not see the tag used anywhere. It's still something many Drupal developers know, and It's important to them, but it's not widely used.
In my opinion we should keep our current "One cart to rule them all", as our tag line, to simply be used among ourselves. Instead of focusing on a better tag line, we should focus on our overall message, marketing, and Ubercart itself.