Also, one Xcode UX idea:
When I started investigating this flag, I frequently ran into the remark that incremental builds were disabled because things were built with different compiler flags. I think this might of happened due to switching branches or xcode versions, but I'm not entirely sure. It does seem to not occur after a clean build though.
It would be a great feature to expose this scenario inside Xcode better so the developers aware that incremental builds aren't working. I spent an hour trying to summarize our incremental build behavior, and once I got this flag and compared the timings, it was obvious that incremental builds just weren't enabled.
Brian