SR071 wrote:
As far as I can see, the only one that can't be duplicated in PS is a polariser.
Even graduated filters can be duplicated by bracketing and merging in PS.
Actaully I've seen a really nice trick to simulate the blueing (if that's a word) effect of a polariser in PS - but it's not as good as the real thing.
Also, no matter how hard you try, you are not going to be able to effectively simulate an ND filter, be it a full ND or a grad. The former is great when you want a slow shutter speed but no matter how much you stop down the aperture (or if you want a shallow DOF) you cannot get it down to the required amount.
ND grads are a landscape tog's dream, as you can correctly expose the land and/or foreground while not having to worry about overexposing the sky.
Personally I use NDs and ND grads (of varying strengths) a polariser and the UV haze for protection. I have a Cokin kit full of silly colours and effects (left over from my film days) but I have to say that I very rarely use them (perhaps the odd sunset filter now and then) but as you say most of it can be done in PS.