I take an AFMU, just to fill an otherwise empty slot, as it doesn't have weight, but haven't used it in over 2 million LY's and over 70,000 systems (or even powered it up). My entire circumnavigation was 635K and 25k systems alone, and only took 2% hull damage, due to my negligence of bumping the throttle while parked at a star. If it weighed anything at all I wouldn't bother taking it.
There you go then - if you have space you may as well take one. Better to have it and not need it and all that.