We had put a few of our plants up to keep them away from our plant enthusiastic cat and gave ger one of her own. This didn't stop her from trying to eat the fake christmas tree. So, depending on the cat this can be really important.

Lilies are so toxic that I think this is important for all cats. Even if a cat has no interest in plants it would be extremely easy to drop some pollen on the floor where a cat would step in it and then groom that paw. A minuscule amount of lily pollen can be deadly.

One of mine knows what "You'd better not be eating that tree" means. I regularly find fake pine needles in the litter box.