I started carrying them when I moved into an exploration Cobra pre-Horizons, mainly because I had the internals for it and saw no downside to it. When Horizons dropped, I switched to an Asp (for that sweet 34 LY jump range) and kept the AFMU for the same reason. I didn't really start neutron jumping until after DWE1, and so my AFMU saw basically 0 use. That continues to be the story today: I use an AFMU extensively when I'm neutron jumping, but otherwise I don't really use it.
Are they useful for general exploration? Sure. Are they required? Not in the least.
I don't do neutron star boosts, but still carry one, in case of an accident (bad landing etc.). It gives me peace of mind to know that I won't have to high-tail it back to 'civilisation' in case of errors. The longer I am out there, the more chance I'll need one eventually.