couple of thoughts on filters
1. I'm currently in the "naked" camp. I used to put the best B&W UV filters on my lenses, especially on those non-Ls that are not sealed. Then I realized that in 30 years not one mishap was avoided with that practice. Recently I did an experiment showing an increase in flare due to the addition of the filter (on my 17-55, which is flare-prone to begin with). So took the UV off and shoot "naked" now whenever possible. now the only filter on my 17-55 is a slim B&W circular polarizer, of course only when needed. my lenses all have hoods -- even my 10-22. that provides the protection I personally need.
I've got a couple of primo B&W UV filters 72mm that I don't use anymore and would be willing to sell :-)
2. yes, you can make minor PP corrections to darken the sky, with all the hazards thereunto appertaining, but what you cannot do in post is remove reflections from water or other reflecting surfaces like airplanes, windows etc. here the polarizer is just fantastic - remember that reflected light is polarized light. So you're at the beach for example -- you can remove that reflection coming off of the water and get nice "depth".
here's a couple of examples. in the first, my objective was to capture the boy's reflection, so polarizer is off. in the second, polarizer is on because I wanted to capture the depth of color in the incoming surf. the dark sky was a bonus but not my primary objective. Even the color of the underlying sand is evident, something that would be impossible without the polarizer. FYI that one is at 17mm (1.6x) just a few inches from the incoming wave. I had to act fast :-)
