Because story time is the one time you're allowed to make some noise!
We started doing this a few years ago. I run through the alphabet each year as a loose way to organize my story times. We start off reviewing the letters we have done so far (catching newcomers up to speed), and I sign each letter as we go. The regulars usually sign with me. Then I ask what letter is next, they tell me, I show them the sign and coach them as they try it. That darn "k" is hard for little fingers! Next I show them a couple sins that start with that letter, words that they might use, and that have something to do with today's stories. I review previous signs as we run across them "Oh, look at all the dogs in this picture! Who remembers the sign for dog?"Now, our story times are for mixed ages, so your babies probably won't give you an answer:) I would definitely add the signs, though - it's one of those "can't hurt, might help" things, and the parents are always very enthusiastic. Be ready with a good collection of signing books to check out!