Ken - you are correct that in nature Plumeria slowly drop their leaves; there may be a sudden major leaf drop with a cold snap but otherwise it is quite gradual, and certain varieties drop more/sooner than others. There is a tree here that virtually never drops its leaves (it is my registered 'Riviera Rainbow'). However, the big difference is that you are digging the plants up, and as such they will be forced to go dormant quickly, and will in my opinion suffer some moisture loss before all the leaves are gone.
I think when shipping plants it will also depend on HOW they are shipped. Some growers ship in the pot or with a substantial amount of soil; I personally bare-root my plants when shipping to reduce weight and therefore cost. I definitely will then remove the leaves - again, to prevent moisture loss.
I hope that answers your question.
