Long time posting in this forum!
O.P. Take it for what its worth but I shoot weddings for a living... Sorry if I "pissed" you off this morning (with the other thread) but it was the truth... And I gave you a tip; Buy a flash.
On with the fun! There isn't "one" reason in particular "why beginners shouldn't shoot weddings"... The lack of knowledge/ equipment is the issue! One can only read so much on the Internet! Experience is what will keep you head above water during a wedding! As I posted in your last thread; I'll list a few more questions/ circumstances that could happen during a wedding! Its easy to SAY that you perform under pressure and its another thing TO perform under pressure!
1. Do you have a back-up camera? If No, You shouldn't photograph a wedding!
2. Do you have a flash; besides an onboard flash? If No, You shouldn't photograph a wedding!
3. Do you have Photoshop? If No, You shouldn't photograph a wedding!
4. Have you photographed a person? If No, You shouldn't photograph a wedding!
5. Do you know positions/ All "typical" wedding portraits desired? If No, Learn them now!
6. Do you have a website of some sort for wedding guests to view the pictures? If No, You're losing money...
7. Does the couple prefer Candid, Posed, or Both?
8. Are you allowed to stand in the isle? If so, when is it appropriate?
9. Are you allowed to use the flash? If so, when can you?
10. Do you know the layout of a wedding?
11. The "Official" says "You may now kiss the bride" And your camera malfunctions... What do you do when your other camera has the kit lens on it? Still use it? Re-shoot the shot?
12. What do you do with a "Bride-Zilla" type of Bride?
13. How do you handle out-of-control wedding party parents?
14. What do you do if you are shooting a wedding when "Uncle Bob" has a D40 and "has to have every-shot" b/c he used to be a photographer for the newspaper?
15. What do you do when the alcohol starts flowing and guests start to get out of hand?
16. What do you do if it rains and your equipment gets wet?
Your circumstance is a little different seeing the Bride doesn't care about photograph quality! But, there are reasons wedding photographers get annoyed with amateur photographers. Lets say Amateur X Photographer gets the opportunity to shoot a wedding! He then misrepresents himself as a professional wedding photographer and messes up the pictures! Guess what; the guests of the wedding will eventually want to see the pictures! Bride X then tells the guests "He messed up and all of the pictures look like garbage! I got ripped off and I knew I should have just used the disposable cameras on the tables!" It puts the thought in their heads that "professional photographers" are pointless and not worth the money! Thus in-turn taking money out of my pocket... Thats why amateur photographers shouldn't shoot weddings!!!
Figs: You're lost... Sign up to photograph a wedding with that mentality. And when you sink, guess who looks dumb? Not the members on an automotive forum that didn't give you tips/ pointers/ advice about wedding photography. You said "A lot of photographers won't let you shadow, so one can only learn by doing." Its kind of like looking for a job! If an employer tells you "Not at this time" are you going to crawl into a hole and live your life in the dark? You will learn 100X more if you are patient and continue asking! This actually works out to your advantage! You will network/ meet MANY photographers! The type you will want to work with in the future, the type you will stay away from, and when you become Florida's top wedding photographer; you will tell your potential clients who to stay away from! Lisa "ALMOST"

had the opportunity to shoot a wedding with me but I was able to make the wedding on-time... Typically, photographers are "worried" that you might take over clients! I think of it this way; If Im doing my job to the best of my ability and someone takes a client away from me, they deserve it and vise-versa!
I hope this short novel helped at-least one person that was interested in shooting weddings! This should not to deter someone from shooting a wedding (photography is awesome if done right) but more as being prepared!!! "Preparation: The key to success!"