You can use a flash pretty much anytime you want EXCEPT during the actual ceremony. After the bride arrives at the altar and the father "gives her away" -- that's basically your last flash shot until they're walking back down the aisle. Everything "during" the ceremony is natural light only. Hence the need for the 70-200mm f/2.8.
A professional owns the right gear -- not merely the gear they can afford. If they cannot afford the right gear then they're not ready to shoot weddings. The flashes are almost more important than the lenses... they get used all day long at a wedding, and usually more than one. This means if you're not already quite adept at flash photography and getting good results, then you're not ready. When I did weddings there were always two flashes... main & side-light. It creates softer shadows when you have to use straight-on flash (which, at a wedding, is probably most of the time.)
Shooting a wedding demands that (a) you've got the all the right gear and (b) you got the knowledge and know how to shoot a wedding. Too many people with a DSLR really don't have either a or b, but want to shoot weddings anyway. The results are, more often than not, tragic.
The "problem" with wedding photography is that you can't do a re-take if the photos don't turn out. If someone were to do, say, senior photos. And the photos didn't turn out well, you could do re-takes and not ruin an occasion. But you can't do that at a wedding... you have to nail the shots and you have to know (not just hope) that you're nailing the shots.