You know, since it was brought up, I think I'm more than set for the Summoner Abyss now, having *7* Summoner Characters with Relics...between that and getting Lulu's Hairpin, the game clearly wants me to do Mage meta...except I still lack a standard Faithga relic. Granted, until Edward's SSB drops, Mana's Paean isn't that much worse than the likes of Garnet's, so I could just have Eiko running with that, and shove like a highly honed Valefor or something on her.
if I were to make a legit Mage Meta team, it'd probably look something like:
Terra (have her SSB, also has the whole "can take a hit and Support 4" thing going for her)
Lulu (Focus is the reason I'm even considering it)
Tyro (Sentinel Grimoire, can also cover things like Full Break)
Yuna (Hymn of the Faith is my best Medica still)
So my options for the 5th slot are basically between Rinoa and Eiko.
I can't really justify any Black Mage over Rinoa (much as I hate her) at this point because not only does Rinoa have a Soul Break, but Rising Sun is also a weapon she can use no one else can and she doesn't have to compete with overs over it. Granted, Garnet's Storm Staff can be given to someone like Vivi, but Rinoa's way more flexible having White 3 and Summon 5, and I'd wager, especially with Lulu's Focus (and possibly a Kefka RW?) that Angel Wing Quake > a self buff. This same applies to Ashe, but Ashe also won't get level 80 in god knows how long anyway, so really not an issue.
The one person who may rival Rinoa here is Hope, thanks to being able to use the Rising Sun, thus has the same advantage. He doesn't have her Soul Break and has lower Magic, though, but he does have White 4 allowing maybe another person to wield Shell/Protectga if necessary, so there's that. Thing is, if I needed an extra person using that, I'd just go with Eiko anyway.
Eiko's competition is primarily Garnet, whom I have the storm staff of, and Aerith, whom I just got her Wizard Staff. Eiko vs. Garnet is a no brainer; extra Medica > modest damage Soul Break, and Eiko has Bard skills, so yeah, obvious whose better there. Likewise, Eiko vs. Aerith is pretty easy for me; they both have identical SSBs now, so it's Aerith having Black 3/Support 2 vs. Bard 4, I think it's pretty clear which wins that. If I really need extra offense from either, they'll supplement with Summons. Additionally, compared to both girls, Eiko can use her own Fairy Flute, so like Rinoa she has equipment flexibility and it's even my best Mind Relic to boot.
ON TOP OF THAT, Eiko brings a free Shellga Soul Break, so bonus points for flexibility. So it's a question of do I want Eiko's general utility or Rinoa's more reliable offense. Worth noting though that while Eiko's Free Shellga is a nice boon, I could have Terra cover that with by equipping Minotaur's Plate.
ALTERNATIVELY!!! I could field Golbez for the added utility, as he'd be a tank and can also use the Minotaur Plate for Shellga if I wanted to maximize Terra's damage or something. This has multiple problems though. For starters, I don't have a 2nd Black Stick for him since he can't use Rods, meaning he's relying on a Yuna or Aerith's White Sticks (which granted, still have alright magic), or the FF6 Chain Flail, seeing as Terra's using her daddy's amputated horn. I don't have a Magic Sword, Axe, or Lance either for that matter. More to the point, like Ashe, Golbez has to wait forever for his MC2, and by then things may have changed dramatically.
So...yeah, between Eiko and Rinoa, who wins probably stems from the demands of the fight. If I don't use Eiko, I'm probably fielding a Kefka's Magitek Infusion or Garnet's Leviathan RW to fill the Faithga niche, ideally the former due to Hastega.
EDIT: Remembered Eiko is still stuck at level 65, though granted, I think she gets her MC2 in not too distant a future unlike some of the characters mentioned above.