For reference's sake, this article is written by Fabien Potencier, the creator of the symfony framework. This guy's probably single-handedly responsible for lifting the PHP community to a higher level, by making great software (which, when he started, was pretty unique, with horrible hacks-upon-hacks like CakePHP being the norm), and by showing people how to do so.
A great part of Fabien's work is that he pushes people onto the latest releases. Symfony2 cannot be used without PHP5.3 which is great of keeping people's servers more up-to-date. Frameworks such as CodeIgniter were guilty of keeping compatibility with PHP4 for far too long. The world of PHP is a nicer place if you are on the latest version and are aware of the latest features such as namespaces etc.
Users stuck on PHP 5.2 will miss out on potential security fixes to Symfony 2. This can potential compound an already bad situation.
I'm not saying Fabien didn't have good reasons, more playing Devil's Advocate.