I believe neighbors' should report suspicious behavior if it is thought that they are truly behaving in a way that may cause harm to the general population, or themselves. This would be difficult to control as there would be many false reports due to personal opinions of others or paranoid thoughts about what people may be doing. The police would also have to handle these claims appropriately and not make false arrests. The major benefit is any possible action that would be harmful to the community can be stopped. Being afraid to speak up ot stop something is worse than not trying to do anything about it.
I don't think informing the police of everything you think is suspicious at your neighbor's house is necessary. If it is a situation that allows you to approach your neighbor's and try to ask them directly about said activity. However, if there is any danger, the police should be informed.
In today's world , where its so difficult to judge a person by their usual behaviour, it might be hard to understand whether a person could a danger to the society. But playing safe is the key. If I have any doubt about a person that could potentially be a threat, I think its best to take precaution and inform the authorities. That's the easiest way to get your doubts cleared and be safe.
Now, not everything suspicious-looking is worth concern. However, there may be instances where it is, chances are you won't know what it could be or what the intent of the person is. It may be in your best interest to have it reported because that could be the difference between something happening to you and you or someone else ends up dying just because you kept silent. Worse case scenario, they report suspicious behavior and it's nothing serious and you live knowing that your fears weren't true, but if they do happen to be true or something even worse would you be okay with not having said anything?