Predation - one organism eating another - is always bad for the organism being eaten because it implies the death of said organism. Herbivory - an organism eating a plant - is a bit more nuanced because plants don't necessarily die when parts of them are eaten.
There are some times when herbivory helps the plant, and this is called overcompensation. For the same reason that a well-trimmed rosebush will grow better roses, a correctly-munched plant can be better off in the long run. On a side note, the idea of overcompensation is a big blow to anti-vegetarian advocates. Herbivory just isn't the same as predation.