Every dependency is one more thing that can go wrong in my mind. I always drop them at every opportunity.

There are some exceptions. Like I really don't want form building to be in the standard library, and I don't want to write it myself.

It is nice when your package manager only takes a short manage to fetch and build though, and you can have more confidence that any mistakes will be your own (under your control) rather than someone else's.

One less thing to worry about. Adds to build time etc.