Each scenario and mood requires a different lighting setup. If a photographer uses too much flash, it will overpower the mood, and look more artificial. However, if a photographer does not light the shot properly, it can come out looking muddy or underexposed. It's this balance between natural light and the use of flash that ultimately defines a photographer's lighting style.
The natural light often sets the mood and feeling of the wedding. The more experienced the photographer, the more prepared they will be able to accomplish amazing light within a very small window of time. On your wedding day, experience is extremely important so that the photographer will be able to create nice light in any environment. It's easy to get a great photo when lighting conditions are ideal, but what defines a professional photographer is their ability to manipulate the light when it's not perfect. A true professional has no excuse; they just get great shots no matter what.