There is, however, a purpose to using baby-talk with infants. It just shouldn't be used with children as they get older and have already developed their language skills to a certain degree.
Infants actually pay more attention when parents use infant-directed language, which is a slower, repetitive tone used in a regular conversations.
How many "baby-talkers" do you know speak slower or more repetitively? Most babble quickly and in a high pitch.
At least, that's been my experience with what people consider to be "baby talk". Speaking slower and more simply / repetitively with infants seems to be essentially the same as speaking "seriously" with them, as you don't use complex words or phrasing with adults starting to learn your language, and you deal with them seriously too. (or, you should be)
Infants are at a completely different stage of language acquisition than 3-6 year olds, so there's no reason to think that there's a single best "essentially the same" way of speaking regardless of age.
http://en.wikipedia.org/wiki/Baby_talk#Use_with_infants