Why do you use 3rd party libraries?

I asked a co-worker why he decided to roll his own javascript remoting framework rather than use something like prototype that would do it all for him. He told me he'd looked into a few, but couldn't find one that just did remoting - they all came more than generously padded with extra cruft he didn't need. I can sympathize with his point of view, but he did get me thinking about the times I've been willing to take on the additional cognitive burden frameworks like Spring, Hibernate, Struts2, etc. add to my projects. Here's what I came up:

• 3rd party libraries are maintained by other people. To put another way, 3rd party libraries are not maintained by me. Which means bugs I get bug fixes for free.
• They're usually the bits of my application that I consider plumbing ie. unrelated to the business problem I'm trying to solve. (eg. Spring, Hibernate) Why am I spending time on problems that don't add value to my project?
  
• Each major revision of a library is more polished/refined than the last. Better abstractions, shifts towards external configuration by xml, shifts back to configuration in code by annotations/intelligent defaults are some of the ways libraries have evolved in recent past to make using them easier.
  
• The very best are used by thousands of developers which makes them battle hardened aka. production ready. Odds are somebody somewhere has run up against bugs I would have otherwise hit in my project. Thanks Somebody!

That's all I've got for now. If I think of any more, I'll add them.