*"Why are so many people around here treating photorealism as the Holy Grail of Poser?" *

IMO, because while it may be unobtainable, it's much closer than it has been in the past. Most Poser users (present company excluded) are probably just now learning about things like global illumination. It's a new revelation and of course they're going to be excited and enthusiastic about utilizing it. 

Higher quality rendering of Poser scenes has long been available, going back to at least Poser 4 when you could export a scene as a .rib file and render in BMRT or export an .obj file into one of the free renderers. Vue has also been an option for quite a while. The vast majority of people were never going to jump through the hoops to use those alternatives though. Now that Poser has the capability, it's opened the floodgates and "realism," implemented with varying degrees of success is what you're going to see a lot of. 

Everyone will find their own level of comfort/creativity with the new tools. Some will continue to look for the last scintilla of realism while others will use it to create a variety of individual looks that reflect their own vision. Give things time to settle down before sounding the death knell.

To some extent, you can "blame" Hollywood for driving the push of 3d realism. As others have stated, that is to a good degree motivated by economics of 3D vs. practical effects. It's cheaper, easier, not to mention often safer to do it virtual. Even there, films like Up! and the new Disney Princess are wonderful examples of traditional or non-realistic looks that still incorporate cutting edge technology in the production process.. There will always (hopefully) be room for a whole variety things and Poser is no exception. Also, the select group of power users (myself excluded) who are the forum regulars don't necessarily represent the entire 'Poserverse.' A good 80% of of the total user base probably thinks IDL is the Israeli army. 

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