Kimahri can be moderately useful if you make him tanker and need a third person to join Tidus and Auron for all-out damage. Plus I dip some White Magic onto him late-game so he can be Paladinesque. Plus his limits can be tough under certain circumstances. Does that make him a Sky Blue Mage?

Edward and Gau, though. Edward is weak and offers nothing. Gau requires a lot of effort to make him even able to keep up with the other characters. Edward is probably the worst overall.