Now I’ve been using POV-Ray for a loooong time before I found blender. It’s a really great ray tracer in my opinion, but the problem is that it’s a very different approach to modelling and shading. It’s much more “physics-inspired” in shading, has a huge library of procedural textures (uv-texturing is a rather recent addition), and, most importantly, works best with solid mathematical primitives, not meshes. So a sphere is really a sphere, not a bunch of triangles shaped roughly like a sphere, and as such e. g. has a clearly defined interior and is smooth at any resolution. Since blender does everything with patch-primitives, half of POV-Ray’s features are already inacessible. Converting textures and shading from blender to POV is a whole other issue, since the texture params and lighting model are very different.
I’ve rarely had instability problems (running on Mac), but sometimes some features don’t render correctly (e. g. always had noise problems with atmospheric media).
The license problem comes from POV being pre-GPL era, they started with a very strict license (e. g. no reuse of code in other apps, no calling of the pov-ray core from other programs, as mentioned) and now they’re stuck with it 'cause the old code is still in there. <EDIT>However, I seem to remember having read something about a complete rewrite (v4.0) quite some time ago (many years)</EDIT> , which should be published with a more “Free Software”-style license. Anyway, this wouldn’t be a problem if blender just generated a SDL text file for POV to render (like it’s done with yafray).
Anyway, considering all those problems, I’m a bit sceptic about the feasability and usefulness of a yafray-like POV-Ray integration in blender.
Now what I’d wish to have is a good export script for individual meshes, so as to do the modelling, posing, uv-unwrapping and stuff in blender, then export and do the scene compositing, lighting, texturing and such in POV using it’s VERY powerful Scene Description Language. This should make it possible to get the best out of both worlds. Still, it’s no replacement for a high-quality ray tracer for blender.