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.
I haven't used DataNitro, but it would have helped me in the past, and it fills a very real need. I think the niche may be bigger than you give it credit for.
These days, I work almost 100% in Python. While I'm not doing analysis much, iPython is awesome for "exploratory programming" - I can quickly build a function to do what I want, thoroughly test it, then import the and its associated tests back into my "real" environment.
The ease of integration external datasources alone makes Python extremely attractive for Excel.
I can easily imagine using iPython to control a small cluster of Amazon instances, using them to process some offensively large dataset (e.g., US Census data), generating a usably small resultset, and dropping it into Excel at the end to make pretty charts that don't look (visually) out of place with the rest of the executive's presentation.
You'd end up with Excel being the GUI of an arbitrarily powerful network of computing resources.
iPython-way: Build or import nice functions, import them, get your (new) data, run your models and see nice graphs in iPython.
So what the niche is that this product focusses on? Empowering the finance-via-spreadsheet world where separate programmers are dying to be allowed to use Python instead of VBA?