Hm I agree.
I just did Ifrit and got 5 weapons but I never tried to do Moogle or Garuda. Even if I get good enough to kill them, there is no reward for me and I don't want to do it over, and over, and over, ... again until it gets really (!) borring.
I rather see some videos of the battles and YES - they are very nice. But it's not nice to kill them after doing a while and getting a dark matter.