Retreading tires is very prevalent in the trucking
industry, as utility
and not vanity
is the motivating aspect. There is no real danger of tread separation
in the case of retreaded tires unless one exceeds the performance and/or tread life parameters
specified by the retreader. Those pieces of truck tread on the side of the highway almost certainly came from trucks who drove too long on the retreads, creating excessive wear, or too fast, causing overheating. A good-quality retread will last almost as long as a new tire, and some truck and heavy-equipment tires are even designed to be retreaded multiple times.
The practice one should shy away from with prejudice aforethought is regrooving. This is where a worn-out tire is cut with a hot iron (similar to a soldering iron) to deepen its tread to look newer than it is. This is extremely dangerous, as it often cuts the tire face to within millimeters of the tire core, and will certainly cause catastrophic tire failure in a very short period of time.