That is certainly an interesting way to look at people talking to themselves, definitely correct with children. Personally I talk to myself a lot when working at home, and never when in an office (it is rude and distracting for other people). I think I do much better work at home, especially when trying to solve problems. For some reason saying something out loud can often make it clearer, easy to see something stupid you've done etc.

In theory anything you can do in a meeting, talking with others, can be done in email, but IMHO well run meetings are a very good way of bringing ideas together, solving problems etc. Talking to yourself is like having a meeting with yourself :-)

Silent reading is a historically contingent practice, and did not become typical until the early middle ages, when reading aloud began to be banned in Christian scriptoriums. Whether this was done for pragmatic or religious reasons, I don't know.

Do you also think someone who draws out diagrams on a whiteboard rather than visualizing everything in their head has a "shortcoming"?

Not everyone thinks the same way. This isn't a bug.