I'm not sure that the default php.ini settings should honored, because I'm not sure we can take for granted that they are intentionally configured the way they are.
The current behavior is to always throw by default, and this looks safe to me while in dev.
Though, if you want to be explicit (rather by implicit with ini settings), you can do:

Symfony\Component\Debug\Debug::enable(E_ALL & ~E_NOTICE & ~E_STRICT & ~E_DEPRECATED);

Now this code does not work as expected https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Debug/ErrorHandler.php#L95
My patch fixes it.

@nicolas-grekas is right. By default, we want to convert everything, but you can override the behavior as noted.

@fabpot then why call error_reporting in setLevel? https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Debug/ErrorHandler.php#L97

it can be replaced by

    public function setLevel($level)
    {
        $this->level = null === $level ? -1 : $level;
    }

The ErrorHandler class can be used by itself.