But she's also a warrior who rarely has to fight any other warriors.
Oh, for sure. My absolute fondest hope with Achilles is that he'll be a lasting martial enemy - BIRDS demonstrated that Gail has some knowledge of martial arts, so it seems not implausible to me that we might get an actual scripted fight here, a regular, powerful physical foe who will perpetuate the meme of Diana as a warrior rather than a brawler by routinely engaging her in weapons combat (improvised or formal) and showcasing blocks, throws, and actual skill. Diana getting to regularly beat the crap out of someone who's a challenge to her other than poor Clark would surely do her good.
I don't see that as a reason to dismiss Poison, though; sure she's still failing to fill that particular hole in Diana's gallery, but that doesn't mean she has nothing unique to offer of her own. So Diana has too many manipulators and not enough meatshields already - well, true. But what nobody in the entire DCU has is a particularly thorough exploration of the intersection of magic and technology, and given how key a concept that is to Diana's culture post-Jimenez, I think that too is a hole in the mythos. She could be the balance to Wacky Alkyone in the long, long-neglected exploration of Diana's culture - if Wacky Alky showcases the social aspects of Themyscira, Poison could reflect further the mechanics and environment those social aspects take place in.