I've been using Filters / Glasses in almost all my shots and trust me its perfect tools once you know how to handle every and each one of them.
Yes, i agree with what the others have said, there is always Pro's and Cons in every thing you do. I always say the tools is only useful when you are expert in using it not because of its build or price.
UV/Clear, for general protection - Yes and it also helps in Street lights to narrow the effect of light. I use Canon UV filter and never remove it from my wide angle lens (16-35mm), perfect for landscape.
It has its Pro's and Con's, not a big user for it because it removes some necessary reflections from the image and overall image looks dull more like mate effect.
However, i have used it in New York time square where the area is covered with LED displays and I really needed it in such situation to remove the glare effect.
So when to use it: I will use in a situation where lights are random like laser show or large stadiums, cars in streets, water, building reflections, events etc.
Yes it is my perfect companion, you will use it in many situations, i shoot in the desert where there are high glare and the sky looks dull without clouds, so imagine all the pictures with sky it shows the yellow empty sky and humid at night.
Without ND filters all my pictures will require extensive editing and yet results will not be good.
So yes i agree with all what the previous feedback but again photography is about different situations and all feedback received may not be relevant because you might be in different situations.
Some one suggested to use bracketing for lighting situation, yes this method can work for a still image, but what about if there are elements which are moving in the picture like a boat or jet ski? this method may not work.
So what do I carry in my kit:
Circular Polarized Filter for 82mm, wide lens - Landscape
UV filter for protection
ND grad with both type for various situations related to landscape
Colored ND grad for introducing color effects
Cons I have 100mm and be warned it will be shown in a wide shot.