> Lockdown mode seems to be one of those features that's great for marketing and PR in certain circles, while being extremely useful in situations where it's needed.

Also, it gives them additional room to play with security research and engineering at large. They already have an incentive to improve security on device (drive by attacks, jail breaking), and this just enables them to play with things that are safer but break too much. They’re basically training their other tech teams to be more secure, and find where security and UX clash, identify and build the fix, even if off by default.