Having additional copies is always a good idea, but you already get that by just installing the modules on developers' machines. At some point you have to trus

Having additional copies is always a good idea, but you already get that by just installing the modules on developers' machines. At some point you have to trust a third party. Even if you run your own hardware, you still depend on power and internet provided by someone else. And unless you are a massive company, time is typically spent much better on other things than hosting your own NPM packages and git repos.