If you like the way they look, I'd get them. I'm not sure if your petstore told you the same thing, but my LFS told me that the moss balls eat the algae and you have to squeeze them every day to let the algae out of them. Well, I did some research and it turns out that they happen to eat the same thing as algae, which can sometimes reduce the algae population if you have a small tank.
So the short story is that I think moss balls can't do any harm, but don't be trapped into thinking that they'll clean your tank for you. Good luck!
-CL95