You know, I use the P4 Engine from time to time still and it really does a similiar job to Firefly in certain cases, and I never use the "Ignore Shader Tree" Option.

The only cases where Firefly is truly needed in P5 is when you want to use displacement or true reflections.  I use Face off's Skin Shader, which is a really complex node set up and it comes out almost exactly the same as in Firefly under the same lighting.  The only difference is the displacement maps, which don't really add much unless you are doing a Close up render.

In Poser 6, you can't use the fancy lighting options of IBL and AO in the P4 Engine, but not all art needs that.  A night shot of a thief in a street picking a pocket (for example) might not require any of that.  If you do a lot of postwork painting, then I'm sure that the P4 Engine is all that you need.

I almost always only use P4 for animations, my clients have never known there might be something better.  (and nine times out of ten they want the thing done the day before they hire me!)

I haven't noticed a significant difference in performance of the P4 engine from P5 to P6, but I haven't used it for anything other than a few quick preview renders.

The nice thing about P6 is that you can do spot renders and save them.  this means you could render something with reflections, once in P4, then reset for Firefly rendering, and just render the reflections, compositing them in later in Photoshop or some such.

Well, all that said, I do have to concede that Textures turn out much cleaner in Firefly, but the final call all depends on your personal style and what you do in your postwork.

Just my opinion.