Upstream, rice it down!
While Gentoo often gets a bad name because of the so-called ricers and upstream developer complains that we allow users to shoot themselves in the foot by setting CFLAGS as they please, it has to be said that not all upstream projects are good in that regard. For instance, there are a number of projects that, unless you enable debug support, will force you to optimise (or even over-optimise) the code, which is obviously not the best of ideas (this does not count in things like FFmpeg that rely on Dead Code Elimination to link properly — in those cases we should be even more careful but let’s leave it alone for now).
Now, what is the problem with forcing optimisation for non-debug builds? Well, sometimes you might not want to have debug support (extra verbosity, assertions, …) but you might still want to be able to fetch a proper backtrace; in such cases you have a non-debug build that needs to turn down optimisations. Why should I be forced to optimise? Most of the time, I shouldn’t.