You can do what you like (as I'm sure you know) but this us a PHP 5.3.0 issue and not a WordPress issue.
Actually, it's both, since WordPress is using PHP and is affected by its changes.
Yes, I agree that we can do what we like, but in this case it would only mean fixing the problem every time a WordPress installation breaks when PHP is upgraded *pointing to the original problem*.
Again, it risks breaking (or at least messing up) existing WordPress installations when PHP is upgraded by someone who is not aware that doing so needs modifying of the
date.timezone directive. Placing the
date_default_timezone_set() function on the core makes WordPress installations more sturdy. When I say putting it in the core, I mean making it available for the next release without us having to modify our WP installations anymore.