In my electrical training there are two things I was taught that were wrong but even a retired electrician, let alone my instructor, cannot give me an answer to why.
thanks for the help in advance
This may help:
http://en.allexperts.com/q/Electrical-Wiring-Home-1734/2008/1/Sub-panel-ground-vs.htm
And here is a good link to why Reverse Polarity is dangerous with a perfect illustration of the lamp socket.
http://googelectrician.blogspot.com/2009/05/what-is-reverse-polarity-all-about.html
One time I was inspecting a vacant home and at the same time a handyman was making some repairs on the siding. I was standing on a balcony above him and said to him not to use the outdoor receptacle just as he was going to plug in his brand new electric drill, telling him I had just tested it and it was Reverse Polarity. He smacked his head with his hand and told me he had burned up another drill just the day before using the same outlet. He said it got very hot and “smoked checked”.
A least 2 reasons.
You do not want the outer screw shell of a light bulb socket at line potential because it is too accessible.
It’s much less likely today but it was once common to have a metal chassis of electronic equipment on the "neutral i.e grounded conductor. A reverse polarity condition would result in a “hot” chassis.
edit: I see Doug posted while I was typing. 
Attached copied from someone…
You could also review these drawing showing the potential problems with improperly wired subpanels.
The problem with connecting the grounded and grounding conductors together at more than one location is that the two conductors will now be in parallel. This will cause unwanted current to flow on the EGC which is hazardous during normal operations. Here’s a good graphic from Mike Holt:
http://www.mikeholt.com/onlinetraining/page_images/1113845784_2.jpg
Good questions and great info guys.
I should hang out here more often! Thanks for the great info guys!
Yes, we need more threads Like this
Ah, just like the good old days, 
before the MB Police. :twisted: