Dusknoir did actually give some nice stat buffs. It brought its Attack up from "meh" to "actually pretty good, at least for the time" and brought its Speed up from "unusable" to "good enough to outspeed walls with investment". It's just that it also happened at the same time that Shadow Ball was made special, which combined with Dusknoir not having claws completely ruined any chance it had at being the tank its stats were designed for.Some evolutions also don't give a big stat increase. *cough*Dusknoir*cough*
Ever since Isle of Armor gave Dusknoir a good STAB move (and a very good STAB move at that) I feel like it's in a pretty good place now. It may not have much to offer over the likes of Doublade and Gourgeist, but it can do what it was designed to do, and that's the most you can really ask for in a series with so many characters.