You might be able to learn from creating the site, but you learn nothing by using a framework, all you learn is how to rely on it. If there's a job opening for a PHP coder for instance, if you've only got experiance in PHP by using a framework, unless that company ONLY uses that framework, you won't get the job. The job will go to someone that doesn't rely on frameworks.
If I have the complete experience to create my own framework for the site, then I'd be glad to switch it from Symfony to my own once it's the way I need it. Until then, I'd have to make-do. I'm hoping I gain complete knowledge to create whatever when PHP 6 comes around (god knows when that's happening).