I don't see the interest in that either...In fact, I always thought that, other then causing a troutload of damage, the particular Summon looks pretty gay..bunch of knights flying around twirling cheerleader batons and King Arthur popping out of a red cardboard box..hmm hmm. Not to mention how long and tedious it is.
Like Eden in FF8, or Sephiroth's goddamn SuperNova move.