Madama LeGuin's article in the New Statesman on the importance of imaginary friends is--ayah--what she said exactly!
I agree on all points. Too sad that once we grow older we are supposed to exchange up our imaginary friends or un-imaginary friends who all too often do not exhibit half the loyalty and faithfulness our invisible friends did.
(Of course, not all of us do give up our imaginary friends...or is it more accurate to say that sometimes our invisible friends don't give up us?)
In fact, if truth were told, I wonder how many adults still do have invisible friends; they've just learned to keep their yaps shut about it...