Filters aren't as critical when taking photos. They can certainly help but you can also adjust your shutter speed on sunny days to compensate.
Video is another matter. I've used my P3 Pro to shoot on some TV shows and commercials. In the film industry almost everything is shot at 24 or 30 fps. The rule of thumb for your shutter speed is that it should be roughly double your fps for the most cinematic look. If you don't have an ND filter or one that isn't strong enough then you have to compensate by increasing your shutter speed which can cause some strobey, noise-like effects in your video.
You also have to be careful not to have a polarizer that is too dark as the native ISO on the Phantom line (not sure about P4 Pro yet) is 100-200. If you go much higher than that you will start to see noise in your dark spots.
Without aperture control it can become a bit of a dance at times, switching filters and trying to keep everything within parameters but it's worth it.
I live in Hawaii and unless I am shooting down at the ocean and want to cut through the glare I don't use a polarizer but on a sunny day I use my ND 16 and ND 32 all the time.
A good set of filters are critical to capturing high end footage but you have to know how to use them and what all your other settings should be.
Final note, it's usually (not always) better to be slightly dark than too bright. It's easier to bump up the brightness a tad and still pull colors out of it in post production than it is to recover colors from blown out, over exposed areas.
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