I have 2 Zone VI viewing filters, both 35mm and 6x6. I noticed they can fetch over $100. I'm wondering if people feel they're really useful. I don't need to visualize composition or framing, but I'm guessing that they might be useful for checking to see if filtration is needed. for example, to check whether the tones of a subject and the sky behind are merging or have enough contrast. Any opinions?