Is graffiti ever worth preserving?

In Bristol, the home of the artist Banksy, graffiti has been elevated to an art-form.

A couple of years ago, the local council there accidentally painted over a mural by Banksy, which was estimated to be worth £100,000.

To avoid further embarrassment, it is letting local residents vote on whether the graffiti in their area should be scrubbed off the walls or kept for posterity.