User blog comment:FairyRaven/Fairy Tail Chapter 328: Zodiac, Review and Commentary/@comment-5170433-20130420032224/@comment-7418318-20130420094147

Just because Lucy can't use it's full potential doesn't mean the spell itself is bad.

The only time we saw it fully cast was when Lucy was already on her last breath. When she fired it at Angel, she couldn't summon any more spirits, but even though she was almost completely drained of magical power it still messed her opponent up pretty good.

It's just like Bluenote told Cana about Fairy Glitter. If the mage is too weak (or has too little magic power left I guess) even an ultimate magic becomes worthless.