The only thing about not using a P.A. is that you tend to get loud onstage. It's one thing to have the wattage available and another to use the power 'all willy nilly' just 'cause it's there. A large church SHOULD have a P.A.
If you can afford to have more power, I say get it. I have an 800w@4ohm Yorkville that I doubt I'll ever utilize that much power onstage. If I need to be that loud, there has to be P.A. support.