I have the Range Boost and I would say that you would want to leave the Range Boost Battery on the bike all of the time.
It is not convenient to remove. It requires the same key as the main battery. Since the bike is electric it the weight of the extra battery doesn't really effect performance. I was originally going to remove the battery when not need but decided it was a PITA. As for waterproof I would say definitely not. The battery contacts on the mount would definitely get wet. I don't consider the main battery or the range boost battery waterproof at all. Look things over carefully and you will see that things would definitely become damp if in heavy rain. The Trek owners manual clearly states not to get components wet. Hopefully future products will be completely waterproof. Not sure why the main battery door doesn't have a weatherproof seal on it.