@Tingsryd,
You're correct - you can define a Plane using the reference axes (trimming or otherwise) - that's one of the few things for which they can be used in terms of defining features but I believe it requires additional geometry inputs in order to do so and that's where I was more or less going - singly, they can't be used for much.
As far as OTHER uses, they can be used for Datum feature creation (Datum Planes, Datum Axes - probably not alone but in combination with other entities) as well as many directional or reference direction inputs where a plane or axis might be used; Measure Distance & Angle (at least up to and including NX11 - not sure what if anything might have been botched with the revamped Measure command in NX12); Assembly Constraints will also use reference axes for certain constraint types and that's about all that I'm aware. This is by no means all they can do, I'm sure.
It's just an age-old question that continually comes up with new-ish users. I wish Siemens would have just made them permanently displayed and maybe a bit more robust. The way they are, they seem to cause more confusion than being self-explanatory.
Tim Flater
NX Designer
NX 11.0.1.11 MP8
GM GPDL 11-A.3.4.2
Win7 Enterprise x64 SP1
Intel Core i7 2.5GHz 16GB RAM
4GB NVIDIA Quadro K3100M