I went out of my way to grind Rikku when I was a horny teenager so that she was on par with Tidus & Auron. I will admit I liked Rikku. Not because she was a good character, but because she looked like a raver girl that I wanted to lick the sweat off of
In the end, it's the Overdrives that give them uniqueness, so you'll always favor Tidus/Wakka/Rikku no matter which paths you're sending them down.
Doesn't this kind of reinforce my point about the sphere grid being horribly designed, especially as it pertains to the rest of the game's mechanics?
If I remember correctly, I hated Wakka's overdrive because it was the hardest one to land perfectly, and it had elemental basis, which is a nuisance unless you're fighting an elemental enemy that has tons of HP (rare). I don't remember clearly the details about his overdrive though, maybe he had a non-elemental slot.
Rikku's was also a bit dumb IIRC because it required expending items. Although I think the effects may be unique and have a wide variety so you may have a point. Tidus is a good, simple & effective overdrive. Auron's target-all overdrives were very useful to me, usually solving the problem adequately without having to dick around with a fucking slot machine or fuck around wasting items with Rikku just to see if it will award me with something decent. And yeah I know, look up the chart-- Sure, but that's pretty gay. So Rikku's overdrive essentially comes with a peripheral girlscout field reference manual, if I decide to cheat.
I mostly relied on Auron, Tidus, and Rikku outside of the annoying forced specialization fight moments. I didn't use Kimarhi once, what a retarded fucking furry character. Wakka looked like a fucking jackass too and his entire character and sportsball shit was unbearable, Im pretty sure he's supposed to be the lower-class Mexican community watch officer who beats on lower-class negroes because he hates them. Was not interested in this King of the Hill motherfucker being in my party even if he was the only one who can kill birds for the first 3/4ths of the game.