Pokemon that are susceptible to Toxic Spikes in OU:
Blissey
Celebi
Dusknoir
Electivire
Heracross
Hippowdon
Infernape
Jolteon
Kingdra
Machamp
Mamoswine
Porygon-Z
Snorlax
Starmie
Suicune
Swampert
Tyranitar
Vaporeon
Weavile
Notable Pokemon in bold.
Yeah, I know that's not a lot, but what it is is notable.
Hippowdon / Blissey / Celebi / Dusknoir / Swampert / Vaporeon
These Pokemon are potent defensive threats, and anything you can do to make them last not quite as long. This is especially notable for Empoleon, as Blissey, Celebi, and Vaporeon are three of Empoleon's biggest enemies.
Infernape / Machamp / Tyranitar
These might not be as immediately important, but if you're running stall, this is HUGE. When you've got one of these monstrous bastards beating down your door, the residual and constant damage from Toxic Spikes is priceless. Often, only having one layer down against these threats is more important than having more than one, as having a flat 12% damage racking up against the foe every turn is HUGE against opponents like Infernape, which take massive damage per turn between Sandstorm, Life Orb, and Toxic, as well as SR and Spikes. Machamp is also big, as SubChamp is vicious, and being able to beat it with Toxic Spike support can be priceless. Same with stuff like CB Tyranitar, which finds itself losing 44-50% of its HP after switching into Spikes / SR / Toxic just *once*.
Toxic Spikes is an amazing tool for both stall and offense to quell the potency of offensive and defensive threats, and it should never be underestimated. It might be completely useless against some teams, but against the things that are susceptible to it, it's second to none.