It's fairly ridiculous that a professional would run out of room on a memory card right when the ceremony is starting. I'm nowhere near a professional, and I can say that when I'm shooting various events I always keep an eye on my remaining photos.
But even then, what about switching to the backup camera? Or why wouldn't you have a spare card on you? Especially for such an expensive photographer.
But having just recently gotten married, and having such a wonderful experience with the photographer my wife and I hired, perhaps I'm just spoiled. :-) The photog even gave me all the RAW photos in addition to his edited JPGs. I didn't think anything of it at the time, but I've come to understand getting the RAW files is not all that common.
It's good to hear you were able to get the shots the photog missed! I was tempted to bring my camera to my own wedding, but I'm fairly certain my wife would have killed me. ;-) Though I've attended weddings where I was asked to bring my camera, and yeah... I might annoy the hired photog, but I wind up getting shots they missed. Definitely worth it!