While snooping around on wowhead looking for some info on the next installment of Ask a Rhino, I found this undocumented(?) change to acid spit.
The damage, cooldown, cost, and duration stay the same, but the armor reduction becomes a percentage, and the stack size becomes 2, instead of 5.
My first reaction is that you can reach maximum application alot faster. My next thought was of Warrior’s Sunder Armor, which with Acid Spit (AS) competes. Sunder Armor is also moving to a percentage, 5 stacks of 4% each, for 20%. So this new AS is just as good, in terms of armor reduction, as sunder armor. What this may mean for the pet’s threat is not stated.
This does bring up the natural question of which debuff is better? Acid Spit or Stampede? All things being equal, and in the absence of other suppliers of the buffs, Acid Spit seems to win out fairly easily, but I think more research is needed!