I always break my eggs into a small dish or measuring cup, instead of directly into other ingredients, primarily to prevent shell fragments from getting mixed in. (And because I have to poke around with a fork to remove the little white strand attached to the yolk, which may or may not be the umbilical cord, but creeps me out anyway.) Some recommend it in case you happen to get a rotten egg (I never have!) Now, I'm thinking the possibility of an extra yolk might be another good reason for doing it, since it could ruin a whole batch of batter.
Have you ever cracked an egg and found more than one yolk? Do you break your eggs into a separate bowl?
You can read more about multiple-yolk eggs here. There's even a picture of one with four yolks! Apparently, the chances of a double yolk are about one in 1,000. But most are discovered in the production process and used in other egg products rather than being sold as whole eggs. These two were inseparable, but there are many images online showing distinctly unattached yolks.