It's a terrible practice; don't do that.
That's why every node project has a file called 'package.json'; it's so you can run 'npm install' and install into your local directory with local privileges.
...I'm really not sure who you've been talking to.
(To be clear this is specifically the way npm is designed to work, and it's very good at it; using npm as a global package manager is flat out stupid; maybe you're thinking of gem...)