Well your changes would ignore the Makefile that gets delivered with the tarball.
Especially parallel build would suffer. And it helps a lot during development to just say 'make' at any point in time.
Why should I change that?

I am not sure that I understand why would parallel build suffer - %make_jobs will create appropriate amount of jobs as would "make %{?_smp_mflags}". During development you can still run "cmake ." and "make" in either local version or in osc chrooted environment. Judging from content of provided makefile it is fully replaced by standardized cmake macros.