This was brought up a few times over the last couple of years, but it would be nice to have this ability to have this exposed on a form. This adds the flexibility to allow users of the form to add what is needed. If it could be turned on when needed that would be great!! Or add another type of single and multi select that would allow this functionality.
I agree that this would be nice!
In the meantime, here is a 3rd-party tool which accomplishes this: