In my opinion, Toxic Spikes are completely useless in today's metagame. 13 (or 15 if you count Espeon & Breloom) of the Top 20 of the OU Usage Stats (PO though) are immune to Toxic Spikes, 1 of them even removes them. And Politoed, Ninetails & Tyranitar often don't stay in long enough for the Toxic damage to matter. If you factor all that in, 18 of 20 Pokemon don't care about Toxic Spikes. That's 90%.
Moreover, laying down just one layer of Toxic Spikes can even be useful for your opponent, because there are a lot of Pokemon that would rather take a bit of damage than be rendered completely useless by a Will-O-Wisp or Thunderwave. Often I have switched my Jellicent (who's one of the few Pokemon that actually really care about being badly poisoned) in when there was just one layer, meaning it would be immune to any other status (including being badly poisoned) for the rest of the game. And the poison damage can easily be healed off by Recover.
In conclusion, Toxic Spikes are useless right now, because
a) too many Pokemon are immune to Toxic Spikes or don't care about it, and
b) you really need the 2nd layer down, otherwise you might just be helping some Pokemon.