But this is the point. There is no good rule for when you should use a library, and when you should build your own stuff in-house. It always depends on the spec

But this is the point. There is no good rule for when you should use a library, and when you should build your own stuff in-house. It always depends on the specifics of your situation. You need to think about things like the quality of existing third-party solutions, and how well they will fit into your codebase. You might find a third-party library that is 90% of what you need, but there just isn't a good way to get to the other 10%, and so sometimes even in those cases it makes sense to roll your own. Other times I see people rolling their own versions of stuff, and it's clearly much worse than readily available libraries. You just have to look at the specific situation to determine the right thing to do.