I have noted over a period that there is a breed of photographer about which is hell-bent on abandoning those tools which have worked well, both practically and theoretically, in favour of short-cuts or make-do methods.
Rob, you might want to check out my other postings: "TS lenses, who needs 'em?" and "what's the use of a tripod anyway?" LOL.
As for myself, I am not one of those photographers hell-bent on abandoning tools which have worked well. I cut my teeth in the world of film and darkrooms, and have thousands of transparencies in my files, so I feel well versed in the world of traditional photographic tools. I'm just opening up a topic that's on my mind right now, and putting my brain on loudspeaker. I appreciate the contribution of Jonathan and others, that's why I'm throwing it out there. I believe rules are meant to be re-examined every so often. If the mighty PF is truly a worthy tool for today's PS savvy photographer then I am confident it will emerge from our humble discussion unscathed.
Back to the subject at hand. I would say in the arena of increasing contrast in the sky and bringing out detail, it's mostly a matter of choice, NOT necessity to use a PF. For reflections, If the subject is not close enough that I want to see into the detail of it, as in wet foliage in the distance of a scene, then the resulting need is to simply increase saturation and control contrast, two things that can be done with great fidelity in PS. Is there a noticeable difference in that approach vs. a polarizer? I don't honestly know. I may need to test that out. I understand what Jonathan is saying about specular highlights, to a point, depending on what angle the light is coming from. But again, are they something I need to see more detail in? A good range of contrast in a photo includes areas too light or dark to see detail in, as we all know.
And let's face it, if your having lots of trouble with glare washing out the color in your shots there's one very old-school approach that works best; shoot near the hours of sunrise or sunset. I know, in the real world that's not always possible or practical, but you've got to admit, the need and desire to slap a polarizer on your lens goes way, way down when you catch a scene in the best possible light to begin with.