reasons to keep dreaming

it's been said that nightmares are manifested directly from the the things a person fears the most. so having a nightmare sounds horrible. but could it be that they serve a purpose as well?
assuming that the above statement is true, a nightmare is as close as we may ever come to experiencing our worst fears. after waking up -heart pounding, sweaty-palmed- don't just shrug off a nightmare as a crazy manifestation that could never happen. while maybe some things dreamed could never actually happen, they may prepare your mind for something equally mortifying.
could it be that dreaming of something awful that you hope will never happen could be a good thing? if your mind has already dealt with the aftermath of said horrible event, then in the case of that very thing happening, your mind would be able to handle it much better.
in a crude comparison, it's like the way television desensitizes a small child. while our minds are in a particularly impressionable state, they're exposed to horrific things, and therefore they can handle it with reduced stress the next time. while it takes a small child a long time to desensitize to something like death, the most effective viewing is the first one. with dreaming, the horrific things we're exposed to come directly from our greatest fears. what better thing with which to desensitize a mind? so could it be that one nightmare could desensitize our minds to a tragedy just enough that, were the tragedy to actually happen, it would no longer induce the potential mental breakdown?
it surely points out the irony in the phrase "in your dreams."
sleep tight.