In general, commands issued in the mongo client shell are not idempotent. (There are a few user-friendly exceptions, like ensureIndex().) This means that, for example, if I want a given node to be in a given replica set, I cannot simply add it – because if it is already present, the add will fail. Instead, I have to get the current configuration, search for the node I want to exist, and only add it if it is missing. This is not a big deal when typing commands in ad hoc by hand, but it is a barrier to automation.