FFVI - Gogo and Umaro are usually the main benchwarmers, I use Gau a bit in the WoB but I do tend to ignore him for most of the WoR until he gets his prize monster A.I. I actually enjoy using Umaro and he's a kick to watch in battle but he's not as good as other party members. Gogo is also just mainly a second tier version of other characters and can't be developed so he gets ignored as well.

FFVII - Since Materia is your party I can honestly say most of the cast is the benchwarmer unless the game forces them into my party. I mainly use Cloud, Tifa, Aerith, and Vincent. Which is funny cause if you saw my rank VII cast, you'll know I don't care for three of these people but Tifa and Vincent's Limit Breaks are at least unique and fun to use whereas the rest of the cast is mainly select command and watch everything die, so I appreciate the attempt at making battles more interesting.

FFVIII - Selphie and Quistis, largely because their Limits are subpar compared to the rest of the casts or can be recreated with the junction system so it's not all that useful.

FFIX - Dagger, Eiko, and Amarant. Amarant just comes too late in the game and he's just not the hard hitter I want him to be. Dagger is a subpar White Mage and only gets her useful Summons late in the game when the rest of my party is getting game breaking skills and weapons, so she pretty much loses her niche entirely. Eiko is a good white mage and an interesting summoner but characters like Quina and the game's customization system makes many of her summons obsolete, and while she's a great white mage, the game doesn't require a dedicated one like say FFIV does.

FFX - Kimarhi, Yuna, and Auron. Kimarhi is much like Gogo, he can never be more than a second tier version of another character in your party. Yuna is here cause she's an awful white mage and Rikku with her Al Bhed Potions is far more useful. Her Aeons are powerful but the game never throws anything at you that really requires them so unless you like playing the game on easy mode she's not a viable option IMHO. Auron is kind of a lie, I actually do use him all the time but it's because I like him, not because he's actually useful because his only strength is not unique to him, once you can get the Pierce skill onto Wakka's weapons, Auron only has one Overdrive that makes him worthwhile and even then it's only useful for boss fights.

FFXIII - Sahz, he's my favorite character but he doesn't have a strong niche in your party once you actually get your whole party.