A polarizer you cannot duplicate in post processing (at least only partially), but a GND you certainly can I do this all the time using Lightroom. In fact you have many more options doing this in pp.
You can browse my websites and find landscape photos that are all done without any physical filter. I use an ND filter sometimes for watershots to make the shutter speed longer, however usually in the light I prefer I can do it without.
You're saying a number of things there.
1.) Polarizers are handy.
Fair enough. I bought a 105mm CPL for the front of my Lee Foundation Kit. I suspect I'll want to buy a regular 77mm CPL too at some point, once I figure out how often I use the Lee Foundation Kit.
2.) You don't like to shoot in light that requires either exposure blending or ND Grads.
I've looked at your work. It's fantastic, at least to my untrained eye. So, I will certainly keep that in mind as I explore the capabilities of ND Grads and exposure blending software. I may land where you landed, eventually, but I have to figure it out for myself. I just can't take your word for it at this point. I don't know what I don't know yet. I hope that makes sense.
3.) You do use ND filters sometimes to get long exposures of water in scenes where this would otherwise be impossible.
From that, I think your original statement was a bit harsh: "Filters are old school, really. I have a few Lee filters but hardly ever use them. In the digital age they are not really needed."
Seems like you really meant: "ND Grad Filters are old school, really. I have a few Lee ND Grad filters but hardly ever use them. In the digital age they are not really needed, but I still find ND Filters useful sometimes."
I'll probably end up right where you are eventually, but I'd like to try ND Grads out before I rule them out. Generally, I really prefer to do as much as possible in camera. I *hate* editing. Sometimes I think I'm the only photographer on the planet with a genuine distaste for editing. I suspect ND Grads will only be useful in a limited subset of situations, however, and I'll have to do the rest with automated exposure blending software:
http://www.luminous-landscape.com/forum/index.php?topic=91800.0