Hi Ethereal,
All of these response are true in my experience. One thing that was not answered for you was whether teachers or higher beings would use this for teaching. It has been my experience that higher/positive beings would not use this as a method of teaching. But perhaps there are exceptions? Like maybe you haven't been able to learn something any other way. I have to believe that my guides and teachers are more creative than that!
If you have negative entities giving you nightmares or other negative experiences a lot, then you could have a portal nearby, or some other way for them to have easy access to you. Some clearing work may be necessary. And there are many methods for psychic protection.
Also, my hubby is one of those souls who travels to negative astrals for "rescue missions". This is beneficial for those souls, but last time he came back with a really bad negative entity attachment that possessed him for a thankfully short time. It was a really freaky and bad experience for all of us.
Blessings,
Keena