Did you check that PhF is ignored even if Darktable is not installed on the system.
Anyhow, this is a real problem… I guess not only PhF is ignored, but also the good old UFraw 
The issue has been already addressed on the gimp-developer mailing list, but there seems to be no big motivation for having other RAW loading plug-ins, or actually just the freedom to choose which one to use.
Maybe we can help me to revive the discussion?
Concerning your patch, I’ll definitely include it, thanks!