On the negative side, Locke is very physically weak for most of the game; the really good weapons for him do not appear until about halfway through the World of Ruin (if such a thing can be conceptualized due to its non-linear nature).

But since you're going to be Stealing with him 90% of the time, and you can usually put him in the party with Sabin or Cyan or Edgar (who can pump out the damage for him), it doesn't end up mattering a whole lot.