DS already has the Renderizer plugin that will allow you to render using Pixie (free), the full version of 3Delight ($0-$1500) or PRMan ($3,500).

I can see kind of  crocodilian's point about resource allocation but they really haven't been reinventing the wheel, they simply bought Pixels3D's wheel along with any limitations it may have. As Stewer says, it would be possible to add support for other render engines but from a profit standpoint, I'm not sure it makes a lot of sense. I tend to doubt that adding MentalRay support would bring droves of Max or Maya users into the Poser fold from the high end and probably not many Poser users would shell out for MentalRay from the low end. From the renderer devs POV, API or not, Poser integration probably sounds like marketing a Ferrari engine for a Kia sedan - not a real big market.

Ironically, many if not most of the people who have the skills to take advantage of a high end renderer are the ones who are clamoring for more power from Firefly. That makes sense - who wouldn't prefer an all in one solution at a much cheaper price and without having to start from scratch?

Either way, I think neither superfly group nor the Mental/VRay group represent the vast majority of Poser users, notwithstanding the prevalence either may have in the forums here. Poser needs to satisfy the majority while preventing the minority from defecting. And where would they defect to at that, since many don't seem to be keen on DS and apart from perhaps Carrara or Vue, they'd have to give up their nodes and dynamics. At this point, it sounds like Poser 8 will do a pretty good job of satisfying most people and Poser 9 will probably be owned by the Chinese or the new GM anyway.
 

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken