While you can, it's not always that simple. Say you want to make a Qt application with a web frame - Qt-Webkit seems to be the most mature for that matter. Spidermonkey seems to be quite a bit behind V8 in terms of performance, tooling and framework support (Node.js ecosystem) as well, so I'm not sure it's a real alternative today. Do you use it for production level serverside-javascript systems?

Also, do you only use older CUPS versions because Apple pays its development for a while now?