Although both of @Jacques Lucke (JacquesLucke) 's usecases are invalid (both are doable without this switch), there is something to be said for discoverability of an option, the option to turn it off is seemingly well hidden in the cmake docs.

I went back and forth in D13797 and ultimately decided against having an explicit option, CMake natively had the option, so why roll our own, seem silly, however given one of our smarter devs failed to find it, what chance do lesser mortals have, I'd rather have 5 extra lines to maintain, and have this explicitly listed in CMakeCache.txt/cmake-gui than have more devs lose time over this.

I would however apply the change consistent with its unity counterpart and use if(WITH_PRECOMPILED_HEADERS) throughout the libraries, rather than the if(COMMAND target_precompile_headers) test that is there now