Oh well, I'm only a beginner who started photography because of the birth of my lil' kid. So far had covered friends', relatives', colleagues' and even fellow forumer's wedding before. Guess I was more lucky that they asked me to help out. One colleague (on a tight budget) got me to be main photographer even!
And mind you, "tan jia" photographers won't be kind to the "helper" photographers. One went to the extend of blocking and stepping on my shoes just because my equipment was bigger than his! ;( I was to enthusiastic and walked in front of him moments earlier.
But if the bride/groom is looking for a different prospective, help-out photographers can capture many (IMHO some are even more "precious moments") journalistic photos, that could be kept forever. For me, I had hired 2 photographers to cover my actual day wedding, back in the film days. One shot in color (standard pictures), while the other in B&W.