Now that Zeps have both AA guns and bomb bays adding them doesn't make to much of a difference in fire power. Putting fire charges in them is basically useless compared to to AA guns. And while you could put them on the bottom and drop tnt 1 its expensive gun powder wise and for a craft that can be so easily shot down very much not worth it. 2 you can already get the same results by placing tnt on any redstone source and letting it fall. (If this is some how illegal and ive missed it then putting tnt in the dispensers and dropping it is also illegal same level of enforcement required.)
That aside the reason I want them is not combat related.
Having a couple of them would allow us to add some fun features to our zeppelins. Namely
1. armor equiper just fun to have and at the moment the only craft you can really put one on at the moment is a truck
2. sf machines with a few despensors you can have some of the basic sf machines on your craft a have a little work shop(For anyone worrying that a portable work shop would be op in some way you could just carry the items and place them where you need them this is just for the cool factor)