I am surprised that there is so much talk about RAID. I have always thought the reason Nikon offer dual memory slots is primarily for video, and also as a benefit to consumers, as buying 2x32GB is much cheaper than buying one 64GB.
If you are just taking stills, it would be nice to have dual slots but not absolutely necessary. How many times have you had a SD card or such fail? And if you are shooting something important, double/triple back once you have finished.