I don't. If you have a bundle doing a crap in its version constraint and the only installable version is the old version compatible with 2.0, this would automatically revert your project back to Symfony 2.0 because composer will think it is compatible, while your code is probably not.
And given the number of Symfony releases since 2.0 and the number of subpackages in it, reducing the range of versions matched at the root level makes a huge difference when running composer (both in term of speed and memory usage).
If you don't want to change it in each branch, at least use ``~2.3``