If you guy View/Pro, /Command, or the SDK, you get a lot more than a "bare bones interpreter" IMO, but that is *my* opinion. I'd rather talk about software than free-versus-commercial issues though. :)
I have both Ruby and Python here, and they're good languages, no doubt. But neither has an integrated system for building GUIs, and I haven't found anything that is as simple to deploy as REBOL (two of the many reasons I like it so much).
I choose (and pay for) REBOL, even though there are free languages out there that I could use. And if you don't need to make system calls, you can use the free version(s) of REBOL. :)
Thanks for writing!