Why ask Sheldon? The instructions that come with a Shimano chain are very specific... do not split the chain at the joiner pin, and use a new one where the chain is split. Most chain failures are identified as the result of human error in rejoining chains.
People can do whatever they like, including using the joiner-links, but if they choose to reuse pins on Shimano chains, there is a risk involved. So long as they are aware of the risk, and are prepared to take the consequences (injury, being stuck out in the middle of nowhere, having to do a roadside repair, loss of a race win), everything's fine.