Up, up and... they did it!

Some events require careful eye and good timing. Can you imagine: "Hey, please baptise her again, I didn't get a good picture"? You can't do it. This is why you should be prepared: flash, perspective, light, exposure, etc., all must be ready for the moment when you take the picture. The faster your camera is between shots, the better, of course.
And everyone who's used a digital camera can tell you that the worst feeling is to be ready to take a picture and have no batteries.