Hacksaw is good opener against air units: The hacksaw can take down initial lone striker, no matter the armor. This can buy a player some breathing room before transitioning into heavier AA.
I use hacksaws to feel like my antinukes and singularity reactors are safe from aerial attacks, if I don't know whether or not the enemy is going to commit into aerial bombardment.
If the enemy gets a bigger airforce, I can easily transition from hacksaw to proper anti-air, be it gunship tridents or spider tarantulas or crashers or anything else.
Overall, the hacksaw makes you less stressed in the early game, letting you focus on epic ground battles before the airstrikes arrive.
(Saying this as a small giant certified)