Instead of "nptl" and "nptlonly" flags using "nptl" and "linuxthreads" would be nicer.
In the latest glibc ebuilds, "nptl" in fact builds both nptl and linuxthreads, "nptlonly" builds *only* nptl. Changing the use flags to what you suggest is in fact misleading and wrong. I'll let the base-system folks confirm this.
maybe at some point, but right now, no