Why use filters? Well good question, however if you are an outdoor shooter, and you like to shoot around water, I can see at least an ND and a CL-PL.
CL-PL to both cut glare off water and foliage and it will also help in gaining a slower shutter speed. I work around water as much as I can, preferring the slower shutter speeds, I need both the use of a CL-PL and solid ND. No software I know of can cut glare off of water/rocks like a CL-PL can, the difference is important on sunny or cloudy days.
Grads, I agree, that you can get a lot done in LR now. Also grads are not going to work well unless you have a pretty even horizon line. I do find the reverse grads a bit helpful at times, (by Singh Ray)
Your 14-24, will be a problematic lens to filter. I love the lens, and carry on all my trips. However the use of a CL-PL is not easy. The only solution I have found is my own modification to a Lee SW-150 filter holder, so I can carry (1) 2mm and (1) 4mm. The 4mm is a CL-PL sheet filter from Cavision out of Canada. This will allow you to use a resin ND filter (so far I have not seen any image degradation in using a Lee resin filter on wides, with Nikon (D800e). You can use the Lee kit that will allow you to use the SW-150 on normal lenses like your 24-70, however on that lens, it's a bit heavy and will make balance a bit hard. The Cavision filter alone is around 400.00. Schneider also makes one, however I find it much warmer than the CAvision version.
The other issue you will have of course with the 14-24 and a CL-PL is that your sky can become problematic at times, since a polarizer on extreme wides will at times create a situation where the sky is much darker on one side than the other. In a perfect situation you want to position the sun at 90 degrees or so to the filter but many scene won't allow that, so you might want to watch for this. You can just rotate the CL-PL to where it's not polarizing the shot, but it still will work as a ND since most CL-PL's darken the scene by 1 to 1.5 stops. This will not be as big an issue with the 24-70 unless you are in the 24-40mm range, but even then nothing as bad as it will be with the 14-24 at 14-18mm.
Paul