It depends on the match up. Something like Golem in neutral weather with no friends needs that 15 ATK for the last breakpoint at 40.
In many other cases, a low IV high level evolved all the way is just as good or better than a high IV low level. And in some cases, only the move + level matter - SC Gengar out performs every other moveset (including Lick by <0.5 DPS), at the same level. 0 ATK SC is stronger than 15 ATK Hex.