As a heavy user of DataNitro in the finance world, I can attest to how much it has improved my life. It allows me to interact with my data providers' excel plugins in a much more expedient manner. I could use their direct data feeds, but testing things on those is a PITA.
Largely, though, I agree that DN isn't really necessary/useful but for a small niche... but when you need it, it's EXTREMELY necessary.