This is ignoring things like Samurai physicals, guns, and swordskills in which is pretty much the only effective support skill boost,
Samurai physicals: Equip Sword or Equip Spear frequently deal more damage, actually. At, say, start of Chapter 3, I'm assuming 8 PA, 70 brave, making Attack Up 120 damage, Equip Spear 144 damage, and Equip Sword 128 damage with a chance of adding Don't Act. (Of course, they could also use something to boost their skillset, like MAU, or ghetto MAU (Equip Axe)).
Guns: yes, unless it's an elemental gun (MAU), stone gun (Equip Change), or you're doing gun+Battle Skill (Concentrate). Though sure: if you're damage-twinking a Romanda Gun, Attack Up+Charge is the way to go. To be honest, I haven't seen anyone go for "hardcore gun damage unit" in Chapter 2-3; if they're sinking JP into a gun user at that point, it's usually going for, say, Auto-Potion or Talkskill mastery.
Swordskillers: Yes, swordskillers definitely want Attack Up. Well...ignoring two swords for Chaos Blade's power and Excalibur's haste which actually deals more damage (but is super-endgame only). I'll be honest, though: I don't want a support to exist just for Swordskillers; I'd rather balance things for generics, and if that makes special characters a bit more tempting, that's fine--most people aren't using them as-is.
It was always a well-used support ability.
At 600 JP I don't think I saw it used ever in the playthroughs Laggy and I did. In vanilla FFT, I used it on Swordskillers and the Geo SCC and...that might be all, actually.
I stuff it pretty often on Ninjas now that MA is no longer the obvious option
That's fair, although I do expect Ninjas to use Concentrate at least half the time. Concentrate makes Throw better, Attack Up does not. Against someone with 25%+ evade (Chapter 2 shields, elf mantles, some front evades) Concentrate deals higher average damage purely due to not being evaded. Furthermore, with the Ninja vulture-like playstyle, consistency has significant advantages. If you know your target is going to die, that makes it much safer to run into point blank range. ...Though sure, Ninjas are happy to switch support skills depending on the fight. They might also pull out Martial Arts and Equip Sword from time to time, as IIRC those two were also competitive damage-wise (at certain points of the game).
I dunno--as a general rule I feel moving everything cheaper is better UNLESS you get overshadowing going on. For instance, Teleport at 600 JP? That really took away from variety in movement abilities. MAU at 400 JP? When the only serious magic competition was 800 JP and became worse in the aftergame due to not working with Draw Out or Mathskill, that reduces variety (and as an LFT-specific problem: why would you ever learn Bolt3 before MAU?) When every character started with enough JP to learn Phoenix Down, and that ability on its own was arguably higher-power than a number of earlygame skillsets, that reduced variety.
Which is to say: I'm not against raising Attack Up's JP cost per-se, but I'd like to see arguments that at its current level its reducing variety. That you would have an interesting choice between two weaker support abilities if it were more expensive, but with Attack Up so cheap you just get that instead. Or alternatively that you might go for something else in Geomancer if AU were more expensive, but instead AU becomes the obvious buy. (The flip side being that, at a glance it looks like Samurai get 3 rather than 2 easily accessible supports to choose from relating to their physical, and similarly for Ninjas).