Many important numerical algorithms like dot products, matrix multiplication, FFT can be implemented in terms of FMA. So yes, it makes sense for processor designers to devote resources to FMA units.

Also, FMA units can be used for pure adds or multiplies as well. Not sure of instruction sets that have separate instructions for addition and multiplication in addition to FMA (like x86), are they all executed on the same functional units or are there separate units. And if executed on the same functional units, what's the limitation preventing only one add per cycle vs. two FMA's as on Haswell.