What is the benefit of this over setting an environment in, say,
/etc/netctl/hooks/environment?
Well, the motivation for this was that I could not find a way how to enable the debugging. Apart of that, using hooks enables debugging only for certain profiles. The conf.d solution enables it globally regardless the profile name.
Just thinking about it, I should have documented how to enable debugging by using the file in the conf.d directory. If this patch has chance being accepted, I will add the documentation.