"full photographer experience" is the kicker here really.  

Can you do weddings? Apparently, because your friend does.

Can you create a dramatically lit portrait in a fully lit, bright room against a bright walls without it...probably not.  


If you always have control over your environment and only want to do certain kinds of shots - then of course you can do it....but you're certainly missing out on a whole dimension of your photography you can control.

Shooting without flash is part of his *style* and thats what he's selling.  There's not really an inherent advantage to refusing to use flash - its just that its helping to define his style.  And having a style - that's an advantage.