Magic is actually useful in all versions of the game. Most enemies outside of some bosses and several marks don't have hp in excessive amounts like in FFX or XIII. So the damage cap is similar to older FF games in the vanilla version of XII. The Zodiac Age removes the limit all together and magic can now break the 9999 limit, though there were several abilities in the original that could do that as well. Even then, XII made buff/debuff magic so important that I feel it's naive when people say magic is useless in this entry. Of anything, it's more viable than in the popular entries.

The real stigma against it comes from the PS2 versions having something called the Effect Capacity. Basically, spells were marked with a certain priority based on their power level so bigger spells tended to have longer load times until execution than weaker spells, but this mechanic ended up causing some interesting munchkin style shenanigans.

Actions like basic melee are not effected by this mechanic so if a mage cast a spell like Flare against a group of enemies that used weak melee attacks, the enemies would likely trounce the mage because their actions don't cost much in the EC tables so they could effectively take their turns even before the player could even if their ATB mechanic says otherwise. Likewise, you can delay an a spell action by simply having the player or enemy cast a bunch of lower tier spells. This is why melee was considered superior and magic was considered useless, but more crafty players learned that this works both ways, and if you had a grasp of it, you could effectively stunlock and defeat powerful opponents by flooding the EC with low cost spells. Enemy trying to cast Scathe? Delay the attack by having your party cast a lower tier spell like Thundaga that not only has it's own long casting time, but has a lower EC cost to delay the enemy spell while your other two party members get extra turns in using melee. Knowing how EC worked could save you a lot of headaches though it can work against the player as well.

The Zodiac Version, thanks to being on modern hardware, doesn't need this system any longer, so it was removed. Meaning players could simultaneously cast spells at the same time in this version. I'll tell you now from personal experience that classes like the Red Battlemage and Time Battlemage are super useful in this game, and Black Mage was a wrecking ball in it's own right with my only regret having been making it Penelo's secondary class instead of her starting class. Of anything, the only mage class that kind of suffers in this entry is really White Mage since several classes can learn White Mage spells through careful planning with Espers and Quickening panels on the License Board. Even then, it's a great secondary job thanks to it's magic buffs, ability to equip Holy Robe for the Holy Boost, and access to the high tier spells.