[Deck Knight]

Acid Spray given these stats is like an offense-oriented version of easing prediction, kind of like Substitute but more focused on increasing costs for multiple switches rather than getting one attack off guaranteed before you have to switch.

Say you have a bulky Pokemon that barely avoids a 2HKO from Fire Blast because of Leftovers. If you Fire Blast, you can't follow up with a KO unless you get a lucky burn. If you Acid Spray, not only does the foe take damage, Fire Blast can now deliver a follow-up KO because of the SpD Drop. Your opponent can thus either switch or make the sacrifice.

As I said in the OP, Acid Spray is not great at generating its own 2HKOs, but it supports every other attack on the set, and if you're facing a stall team, Acid Spray wears down an opponent quickly. Acid Spray's damage output geometrically increases as it hits the same target. If it does 60 damage the first time, it does 120 (-2) the second time, 180 (-4) the third time, and 240 (-6) for all subsequent uses if the foe does not switch out. Combined with the fact Dry Skin's Rain healing is 12% and Life Orb's damage is 10%, it's quite easy to run a Rain Life Orb set using Acid Spray to soften opponents up and your other moves to finish. It's actually an exemplary use of STAB, since ordinarily Acid Spray would be like a 100% Accurate, Taunt-proof version of Fake Tears. STAB and 131 SpA make it a threat in its own right to stall teams, and an offensive supporter for every other move on the set.

In this case its primary means is damage: It brings foes well into KO ranges for all its other attacks, in addition to breaking any stall stalemate it would otherwise be up against. Blissey walls plenty of Special attackers with 130 or so SpA, it can't wall Mollux, and if it switches out after the first Acid Spray, whatever comes in next isn't getting in for free either - unless it's a Steel-type, which plays a dangerous game of coming into STAB Fire Blast in just such a scenario. It's quite elegant in my opinion, and adheres to the concept beautifully.