Would pumpkin spice be good to add to cat food or not? If not why not?
Short answer, NO. Pumpkin spice is not good for Cats, Dogs or Rabbits, or probably any pet.
Plain Pumpkin is fine for most any pet. It can be helpful in digestion and as long as it not spoiled is fine for cats
Pumpkin Spice on the other hand can contain any number of additives (spices) which can be toxic.
There is pumpkin specifically canned for pets, but you can also buy canned pumpkin at most grocery stores. Purchasing plain pumpkin, is fine, read the lable to verify there are not additives, but NEVER BUY PUMPKIN PIE FILLING it has the spices that can harm (or kill) your pet.