However I belive that there are many who are not in such a position, but do issue a big cautionary warning to those looking to jump into the deep end of the swimming pool. Yes some people are going to swim well and come out on tops - but more are going to flounder and possibly drown (fail) in the attempt.
Fact is that a professional job (be it a wedding or a modeling shoot or product photography) has certain requirements of both photographer and their kit to get things to come out right. When you take something like a wedding - a one off event - there is huge pressure on the photographer to perform above and beyond their best to get all the required shots. Starting from scratch because you can take some good profile shots is a very big risk - ignoring the chances of kit failure - there is the whole topic of lighting, weather, space, time and more at the wedding. I think many try to impart this warning on newer photographers to discourage them from making a fool of themselves and ruining the day for a new couple.
That can lead to friction and loss of friends as well as (at the worst end) lead to legal action being taken.
I think that's the right answer. Someone comes onto the board and says, in essence, "I just bought a hammer -- I've used it a couple of times, and everyone says the nails I've hammered are pretty straight. So a friend of mine wants me to build his house -- should I do it?"
And the answer is "Nooooooo!" Building a house requires so much more than just a hammer -- in both experience and equipment. Do you know plumbing? Do you know architecture? Do you know electrical? Do you have a ladder and a cement mixer and a saw? These are things you have to know and have before you even
think about walking out there and building a house -- for pay or for free.
The comeback to that is "well, he's really just looking for a shotgun shack, and he can't afford anything else. So if I don't do it, no one will." But the prevailing view is that although people will say up front that the quality doesn't matter, when they move in and the house doesn't have a roof...they'll be upset. (And that saying "but I told you I didn't know how to do roofs!" won't placate them.)
Taking a camera and doing wedding photography is the same. You don't know lighting? You don't know posing? You don't have fast glass and good lighting gear and backup bodies and all the other things that you need? If not, it will be very, very difficult to get even basic-quality shots -- and although people say they don't care about quality, they're going to be miffed when every picture is badly underexposed, or the only shot of her saying "I do" is blurry and cuts off the top of her head, or when you're standing in the way of everything during the ceremony.
But, that being said, there are people out there for whom an amateur photographer is really what they're looking for -- a friend of mine got married in front of two witnesses, and I snapped a few shots with a pocket digital at her request. Did it matter that the pictures weren't great? Not in the least -- it wasn't an event for her (I mean, she was wearing jeans, for goodness sakes), so even if all of the pictures had been blank she wouldn't have minded. If that's truly the case...snap away -- some pictures are better than none.
But be careful -- particularly if they've gone to the trouble of getting a church, a dress, etc. If so, expectations are likely going to be higher than they might let on, and that's a good way to find yourself in a jam -- I think that's what people on here are trying to warn others about.