Interestingly, my older child was huge on imaginary play but rarely did it *with other children.* It was usually a solo activity, where she would talk for the animals, dolls etc to herself but not involve others. The exceptions is/was her brother--I think she is able to trust him enough. Sometimes she would do it with us, too, but it wasn't of great interest to her to do it with us. She definitely has a rich internal imaginary world, though (these days it is channeled into creative writing).
Anyway, I actually thought of this as another sign of her mild social delays. Imaginary play with other children is quite challenging. There are no rules and you have to collaborate very intensely.
I think imaginary play is really important. There are probably other ways to achieve some of the same ends, though.
