Calling addOutput on an Output node uses a registry that is currently populated by startup/gui/outputs.py - however it is quite reasonable that someone might want to call this function from a batch script, without the gui configs loaded.

John says:
"Yeah, it doesn't seem great in the gui startup. I guess it could be split into separate startup files for GafferScene, GafferArnold etc? I'd be a bit wary of this breaking something in a pipeline script somewhere, so maybe for 1.6?"