Here's a TV Forum dilemma: the music video for Wham's Last Christmas which is 16:9 letterbox.
When it's shown in its original aspect ratio, it's postage stamped and looks a bit odd.
When it's cropped, the limitations of the picture quality are enhanced, especially on modern sized TVs.
When shown in its entirety I'd usually chose to leave it in 4:3 - and pillar boxing if it is not possible to broadcast it 4:3 full-width (there is no HD full-width standard)
It's a case by case thing though - if you can upscale using a high quality upscaler in an edit (not just the default NLE scalers which are often lousy) and are only using an excerpt in the midst of other 16:9 full-width content - there is an argument for cropping and zooming to 16:9 full width.