I hadn't heard this before and did some googling on the topic. Apparently there have been studies done in humans where it has helped with gingivitis issues by boosting the immune response in gum tissues.
That being said, I've had a few cats over the years that have had different forms of stomatitis. While all of them point to immune deficiencies, what triggers the deficiency varies depending on the underlying disease. Stomatitis is a generic term for many gum diseases. Therefore, I suspect that the immune boosting properties may help some cats, it is wrong to think that it will help all cats with the disease.
This is one where I would turn to a cat specialist for guidance. I suspect that most vets don't have the time to stay close to current research on the topic, and this is one drug that is clearly not fully understood.
