If you get a couple of good skills on Quina, then Quina is a beast during many fights. Personally, I see Quina as a Red Mage role - able to hit decently, able to do cast decently and able to heal decently. I would use Steiner/Zidane/Garnet/Quina whenever I was able to.

Quina is also kind of hilarious and/or sweet due to how simple Quina is.

Skills I felt worth getting...

LV5 Death - Obvious spell is obvious. MP cost 20.
LV4 Holy - Ditto. MP cost 22.
Aqua Breath - Quarters enemy HP (50% accuracy). Good for high HP enemies. MP cost of 14.
Mighty Guard - Protect & Shell on entire party. MP cost of 64, but still worth it in some battles.
Matra Magic - Reduces enemy HP to 1 (20% accuracy). Great for Eat. MP cost of 8.
1,000 Needles - 1,000 damage. It has it's place. MP cost of 8.
Pumpkin Head - Damage to a single enemy equal to the difference in Quina's Max HP and current HP. I've used this to great effect on occasion given it's very low MP cost of 12.
White Wind - Recovers HP to all targets equal to 1/3 of Quina's Max HP. MP cost of just 14 makes this really useful.
Angel's Snack - Uses four Remedies from your inventory on the four players of your party. This has an MP cost of just 4 and if you're like me and stack up on Remedies etc. due to the over-abundance of gil in the game, then this is a no-brainer during fights that use a lot of status effects on all of your party.
Frost - 30% chance of giving an enemy the Freeze status. Cost of 8 MP.
Auto-Life - Target is auto-revived the next time they get KO'd. MP cost 14. Not majorly useful but like many of the above, it has it's place.

Not to mention Quina is pretty strong for most of the game when doing standard attacks, although for some reason in my playthrough Quina's attacks struggled near the end. Not sure if that was my fault or not.