Generic Atk buffs fall off because they stack additively to character side (ie same as assault).
And considering that we largely measure progression through assault value, you're gonna see that +10%/20%/whatever make less relative impact for you the stronger you are. Like if you're at +100% assault (ie your character-related multiplier is ~2.0), a +20% character atk buff raises that character multiplier to 2.2, or a 10% increase. Compare that to when you're just starting out and have ~0% assault, so a generic 20% atk buff actually does increase your damage output by ~20%.

And yea, the turn-based duration doesn't help buffs, relative to debuffs.

Defense buffs on the other hand still retain their relative impact for the most part. It's just that they're mainly limited by the turn-based duration. Still, possessing defense buff(s) is always handy.