Some of these are pretty wrong, actually. Gau is actually one of the most ridiculously overpowered characters in FFVI, to the point where he borders on being a gamebreaker; you just have to know how to use him. Relm is pretty useful too; she has the highest natural magic stat in the game (although by the time you recruit her you'll probably have raised Celes and Terra's magic stats to comparable levels with espers). And Umaro is useful in the Colosseum if only because he's the sole character in the game guaranteed
never to do anything stupid there. Gogo is difficult to make a case for, though, as (s)he's a jack-of-all-trades but master of none.
As for the other games, Yuffie is, if memory serves, only of only two characters with a healing limit break, which meant she basically never left my party after
(SPOILER)Aerith's death. And Garnet is pretty useful as a healer too.