An external standard (de facto or not) is always better when dealing with things like this than a internal API. If I want to reuse that daemon for other things I know what to expect and how to operate it indipendently.
I disagree it's always better. It's a tradeoff. Now to install apache securely you need to go and install another daemon under a different user and configure apache to use it. How many people will actually bother? If it's builtin and driven by apache itself you just need to upgrade apache. So if you want mass adoption of this solution in HTTP servers a builtin solution works best. If you want to be able to reuse this for your openvpn/xmpp/etc server a separate daemon is best.