Whatever its faults, the advantage of this:
over the 8640B is
portability, which I have been thinking about a lot since I started haunting equipment-filled garages, looking for interesting bits of kit to acquire. I've assembled a small bag of gear for basic taking-apart and testing but I was stuck for a decent signal generator. Then I remembered this:
My trusty, battery-powered, good to 500MHz (more or less) CAA antenna analyzer. It puts out a 400mV sine (more or less) wave over its frequency range and fits in my bag. Not a function generator but for simple testing, good enough.
...and somewhere around here I have both (HF and VHF) of the pocket-sized Autek RF Analyzers, too. If I can find them, and they still work, I can use them instead and get the bag smaller, still.