P3P selling more than P3A is only because people who are willing to spend $1000 will spend $300 more because they can to get the latest even if they don't really need it to make a viable difference when in a years time, when 4K becomes more standard than it is today, THEN maybe buying a 4K capable quadcopter will make a difference.
It's their $300 to spend, no reason to get judgemental about it.
Filming at 4K and downsampling is the only way you can get 1080 video at 4:4:4 (maximum color resolution). Filming at 4K also allows you to crop from the best part of your scene and end up with a nice 1080 video with no corner distortions and perfect framing.
For photography, there is no difference, it's same 12MP sensor. The only difference between the A and P is the extra hardware that does the 4K/UHD video encoding. I have a friend that wanted one just for photography so I recommended the P3A and they are thrilled. Every circumstance is different.