Well, you got one thing right... its ENTIRELY personal. I've done 3 so far with ONE camera and done so successfully. As I said, if you plan to do this long term, it is important to have backups for a couple of good reasons. If you are doing it to find out if weddings are for you and doing it as a second shooter, its entirely optional. And PERSONALLY, thats the way I did it three times already without issues (just got a call this morning that it may soon be 4 times, and I am not getting a backup camera, thats guaranteed.

).
Not having the lens won't let you get the shot, but there is nothing anyone can say to me that if you are
trying it out, that you MUST have a second body. There is no law that says you MUST. It's just recommended as a safety or convenience. Not sure how you shoot, maybe you need a backup. I sure did not for what I was doing. If I was getting paid for this, sure, no prob. I am not getting paid for this, and I am not getting a second body for it. Strange... the client and photographer neither have issues with this point, I wonder why you would?
If you think that all people that are going to shoot as seconds for reasons of experience, fun or just to see if it is something for them... ABSOLUTELY NEED a backup camera... I am about to disappoint you... yet again.