My philosophy is that you should only use something when it solves a need that you currently have.
There's a real tendency, particularly in this realm, to simply use a framework for the sake of using one or because you feel like you should be using something. I discourage such practices. Also frameworks I think have the most value when you've done things the hard way so you have a good understanding of what problems you have and how the framework can help you in that situation. Framework choice does depend on the situation.
That all being said, the MVC pattern with Web applications is useful so it certainly wouldn't hurt you to learn at least one. I'd stick to a fairly minimalist framework however. CodeIgniter springs to mind here.
Lastly, the other danger with frameworks is that they can be so invasive that you're no longer doing PHP, you're doing CakePHP, Symfony or whatever. That's not necessarily a problem but the danger in it being so invasive is you don't get a good grounding in PHP and if you ever want to change it'll simply be too hard that you'll never be able to do it.
