Asking 'should I use fanatics ?' well the answer is...
YES - always !
The are too good to be left out of your army.
The downside is that the don't always work (if you're unlucky)
The upside:-
D6 str 5 hits -3 armour save and possible panic test (if 25% casualties)
The average enemy will have to waste about 70-100 pts just to counter your fanatics. Usually some skirmishers, light cavalry or flyers to trigger their charge and pull them out of their host unit.