The other thing to remember (which JOE! mentioned) is that whereas both STABs of Ghost/Fight are COMPLETELY walled by its counter (i.e. assuming the Pokemon has STAB/STAB/non-offensive move/non-offensive move), Dragon/Fight can at the very least hit it's counter for some damage (even if resisted).
So in a situation where it's Ghost/Fight and Dragon/Fight versus their respective type counters as the last Pokemon on both players teams, assuming STAB/STAB/useless move/useless move on both, Dragon/Fight will at least be able to put up a fight, and if their last Pokemon is weakened, maybe even win.
Dragon/Fight is simply a strictly better offensive typing than Ghost/Fight, even ignoring movepool, stats, and relative metagame (in which case Dragon/Fight would LIKELY be even better).