Copy-on-write semantics can be pretty useful. You've built up a large in-memory graph and want to execute a destructive or long running query on the data set i

Copy-on-write semantics can be pretty useful. You've built up a large in-memory graph and want to execute a destructive or long running query on the data set in the background (e.g., write it out to media when the main process MUST continue to service requests), you want to generate a snapshot core without exiting, etc. etc. I don't classify these use cases are concurrency-after-development cases. Lots of systems use this to do interesting things.