I have always assumed that single use meant, on a single chain. This makes sense as both the chain and link will wear over time. Therefore do not reuse on a new chain. Perhaps I have this wrong, but I've disconnected and reconnected the same link on a chain for the life of a chain. Probably a few dozen disconnects and reconnects for various reasons. It would be completely absurd to expect to literally only snap it together once and apart once and then not reuse it.
Common sense suggests that if it doesn't actually snap together anymore, then it should be replaced. But I've never had one that didn't still have a positive snap when reconnecting throughout the life of the chain.
Also, in all the things I've read in forums like this, I have never once read where a master link failed in any way. Not saying it hasn't happened, but I've yet to hear of it.