1. They let novices build fairly effective user interfaces, just by entering headers for rows and columns.
2. If I understand what "data flow" programming is (based on using LabVIEW), then the formula cells in spreadsheets are such a thing. Each formula is recomputed whenever its inputs change. That creates an incredibly quick testing cycles. Displaying intermediate results by necessity creates a built in debugger.
In addition, the lack of "complete" programming features, such as loops, may make spreadsheets less forbidding for novices to create and debug.
If these things are, as I suspect, compelling enough on their own, then like you say, the rest of the stuff in the latest Excel would just be icing on the cake -- not that I'd give up any of it.