if you use ``~2`` in yoru project, it means that the lower symfony version supported by your project is 2.0. This will be wrong most of the time (you cannot have a project compatible with 2.0 and 2.4 at the same time when you use forms for instance).
A new application is likely to rely of some new Symfony features.

On a side note, using ``~2.4`` in your root package rather than ``~2.0`` will make ``composer update`` faster because composer will never consider any of the symfony component versions below 2.4 when resolving the dependencies (as they are forbidden by the root) even when a package asks for ``symfony/yaml: ~2.0``. Given the number of Symfony releases available on packagist, thus saves quite some time in the dependency resolution too.