We are told that one reason people get "stuck" in mental planes after death is that they truly can't see spiritual helpers trying to help them.  This is why, according to TMI and Bruce (and others), people incarnate on earth can be valuable - assuming that the "stuck" soul is thinking in an earthbound way, such as not realizing he or she is dead.  They would see "normal" incarnate folk more easily than heavenly helpers.

I have read accounts where people who are more self-realized, in a "higher plane" can see both those on earth and in stuck levels, but the reverse is not true.  Swedenborg says that when or if a person is brought into a heaven when he or she is not ready (not loving), they see blackness or nothing.  When a loving soul descends to a hellish realm, they are often confused with the negative emotions, and dark thought.  Thus, interaction between people in various stages of spiritual progression is, difficult.

Earth seems to be a functioning world where consciousness can exist across a wide range of spiritual development - in a level playing field.  People who are unloving - not spiritually advanced, can see and interact with others who are truly loving and giving.  If those who are troubled can have this advanced contact on earth with "angelic" souls (who are also in physical bodies), perhaps this is one of the greatest opportunites to advance the consciousness of a large number of souls - to plant the seed of love, which is difficult to plant in the mental planes of the afterlife. 

I marvel at the coexistence in the physical world of such a diverse group of human beings and consciousness.  I suspect that there is no other plane where spiritual beings can interact together in this direct manner.

Matthew