Do Cats Only Meow at Their Humans?

Meowing Cat by Vera Kratochvil

After reading Adult Cats Only Meow At Humans by Michael, I haven't been able to get this question out of my head.  How come my cats are very vocal towards me when they want something, but, don't ever seem to be vocal at each other?

What is the reason they were given the ability to make so many different variations of meowing, if the sole purpose was to communicate (demand) things to their humans?

Have we kept our domestic cat friends in a kitten stage by babying them?  Are they requesting things from us, as they would have with their mothers?  At what point are kittens suppose to stop meowing at their mother? 

Dr Morris’s theory states that people keep cats in a permanent state of kittenhood in respect of the human-cat relationship because we act as a surrogate and permanent mothers.  This must be the reason behind the vocalisations.

The ASPCA's theory is that cats are meowing when they are requesting something from us.  Maybe cats don't ever feel they need anything from another cat.  However, my cats are perfectly fine with telling a cat when to back-off.