Snails can be bad news. They can be hosts for numerous parasites and even get them from birds and pass them on to your fish. If you swim in your pond, they can give you a nice dose of swimmer's itch.
My favorite bg/sf, the RES. Shellcrackers at least keep them down if not out. Although I think there is a place for every critter in every pond, I'm not a big fan of snails. I agree with JHAP. You just can't get too many RES.
