That's not strictly true. They cut out reflections such as those that cone off the sea. You can get some cool see-through water effects with CPLs. They do work on UWAs too so I don't know where you got that impression.
NDs can add interest to bland skies by giving interesting gradients to blues and clouds.
I don't think NDs and CPLs are always about maximising DR and turning skies bluer, they are extra tools you can use to create something a bit different.
For someone new to landscapes you certainy seem to have all the answers