Is it worth it to replace old style breakers with GFCI or AFCI breakers?
It's probably a good idea for both.  A GFCI measures the difference between current into a circuit on the live wire and current out of it on the neutral; the ground wire just helps to make sure that in an electrical fault, the current isn't passing through you to ground.  So a GFCI would be good in the places that current code calls for them: kitchens, bathrooms and outdoor circuits.

Same for the AFCIs: they help detect faults due to damaged wiring.  With BX cable, that damage is less likely to be caused by driving a nail or screw through the cable than it might be with NM cable, but you still have the possibility of loose connections in your receptacles, or damage to electrical cords going to your appliances.

Another thing to consider: if you have older wiring, you may also have older breakers.  If you're replacing some of them with GFCIs or AFCIs, you might think about having the rest of them replaced with newer breakers.