Toxin damage ignores shields, making it effective against Corpus Flesh. With a guaranteed 100% proc of toxin on every hit on top of the status chance proc, applying stacks of toxin can be effective in some instances. Also remember this is a video game and the Grineer in the story at least are not always attacking military installations of the Corpus. Against less defended or civillian targets these would be quite useful.
As Neithan described, this is quite useful on the Corpus (but not on the Infested, obviously). This is more likely so in the case of those who are not part of the three superpowers, such as resistance groups. Weapons in the game are not solely created for gameplay purposes.
Just imagine the Acrid needle being inflicted on a normal person. The needle hits them in the arm, which then starts to erode the victims skin as it injects poison into their bloodstream. Eventually, the burning sensation makes them panic, if not outright make them go insane. And the rest is up to you to imagine further.