Dave,
I kind of wondered the same thing. I came to the following conclusions, though, that (for now) seem to have satisfied me:
1) Since the GeometryService doesn't really do anything except pure geometry computations, I felt it probably wasn't necessary to secure it using, say, token-based authentication. It would be one thing if the geometry service exposed my data in some way, but it really doesn't. Because of that, I didn't have any issue leaving it unprotected.
2) My Editor is associated with a Feature Layer, which I definitely want to protect. I know I can do that by setting its ProxyURL property, so I'm good to go there.
Since those are the only two services the Editor uses, I was fine with simply securing the feature service, leaving the geometry service as-is, and going from there. I'll make the conjecture that the Editor itself probably doesn't require a ProxyURL property, since all you need to do is provide the proxy URL to the feature layer, and the geometry service, as you have, if you need to do that.
It would be interesting, however, to get ESRI's take on this.
Thanks!
Brooks