I had both Dagger and Eiko on at all times, which I found extremely effective. Dagger’s main purpose was offensive magic, using Bahamut enhanced by additional garnet stones. Eiko couldn’t get over 6k in damage, but her purpose was healing, something she did much better thanks to her Trance skill and available spells, not to mention the random chance of a phoenix revival.
Comparing Quina and Vivi is laughable. Quina’s repertoire of offensive magic is ignorable, his/her greatest assets are Mighty Guard and White Wind, making him/her a support caster with actually decent melee offensive/defensive power. In general, making Quina the first Blue Mage in the series to be worth shit since Strago, and certainly the last (What is even the point of that class in FFTA?). Vivi by comparison is a purely offensive caster with no melee.
Amarant and Steiner have absolutely nothing in common either. They are both based on the templates for Monks and Holy Knights from FFT respectively, and act as such. Amarant has lesser healing abilities and status ailment attacks, countered by a comparatively shitty repertoire of heavy-damage skills, making him better for regular dungeon crawling than heavy boss fights, unless you can procure stuff he could throw. Steiner was a purely offensive fighter who had the easiest time achieving maximum damage thanks to Shock, at the cost of high-MP usage.
Zidane’s role is during standard play, where Steal and that ability that allows you to shift to pincer formation make regular battles and gaining new skills much easier. Soul Blade’s usefulness depends entirely on what weapon you have, but it could have a number of useful effects at times. Thievery is the only offensive skill he has that makes him viable against the big bosses. Freya on the other hand is more similar to Steiner, except she can cast Regen on everyone and regenerate MP. Her usefulness against big targets depends entirely on Dragon’s Crest.
Basically, the only characters I’d even consider comparing would be Steiner and Freya since they are both heavy-hitters with relative easiness (You’ll probably train killing dragons anyhow), and in every case I preferred Freya thanks to her lower MP consumption and availability of white magic. The rest have nothing in common, save for Zidane’s occasional ability to cause status ailments like Amarant, via Soul Edge, which wasn’t the point anyhow since Amarant’s big thing was Chakra.
As opposed to FFVIII, where you literally have nothing to do but “Aura->Limit Break”, every character in FFIX had at least several very useful skills, using just “Fight” in any even remotely big fight would get you creamed. And I hope you’re not claiming that any other game in the series save Tactics made any bigger attempt at making regular gameplay tactical instead of stat-reliant, because that would be absolutely hilarious. Every single Final Fantasy game has either a couple infallible IWIN buttons coded in, or just has you throwing generic indistinguishable offensive stuff at an enemy and occasionally heal. Final Fantasy as a series has been notoriously abysmal at making combat anything but a monotonous chore.