Sorry but your mistaken. It's happened to my tanks before, and it's happening to one now. They CAN eat plants and kill the ecosystem, they can also be just fine in the tank. But in my experience, they always harm, not help.
I mean no offense by this but if it appears that they are harming your plants then your plants are likely not doing well and the snails are taking advantage of decaying and malnourished leaves. Pond snails and other common snails like MTS have been a natural part of planted tanks for decades.