It's fine
Most people here are thinking of the benifit wrong. Duct tape doesn't need to offer much protection to prevent you from getting a flat. That's what your tire is for. Duct tape only need protect you from the tips of the thorns or whatever, while simultaneously moving your inner tube out of the way.
As for safety, and the glue. Many bicycle shops use Ductape as a rim liner on specialty bikes (those with atypical sizes).