It really depends on what you're doing.
Also, I use SUMPRODUCT way more than I use SUM. Almost everything I count or sum is conditional...and there's usually more than one condition..."Give me the sales for company ABC for the month of X and year of Y")
And some people might use a Pivot Table for this and never use SumProduct()...
I use SumProduct() a lot, more that your average user to be sure, and I'm a big fan, I think it's wide array of use is under-rated...
But I can't seem to apply SumProduct() anywhere near, and I mean nowhere, near as often as your plain-old, vanilla Sum(). For whatever reason, I just seem to be summing lots of data...
It seems to me that Sum()'s not secondary, it's primal. It's one of the basic building blocks for all Worksheet Functions in Excel, in practice and conceptually, no?
But, yeah, it depends what the task at hand is. If you're doing a DCF model, IRR() might just be the best Worksheet Function ever (if it works), 'cause that iterative, hit-or-miss calculation is a nightmare to do by hand.