"But if the choice is between writing something yourself and using someone else's well-tested, heavily-used library, always go for the latter."

Absolutely. However, there are plenty of situations where what you pull down from npm or rubygems isn't actually all that well-written or well-tested.

When I first started programming I kind of had this impression that if an open source library is published on a package repo and people are using it then it must be much better than anything I could write. I have learned the hard way over the years that is not always true.