This is a very interesting topic. I think the most important factor here is the 'emotional side of trading', many times people tend to make mistakes when using a real account, mistakes which may not necessarily happen if they were using a demo. This is due to the fact that there is added pressure put on the trader and added risk since he/she can now lose REAL money.
In short:
DEMO = Nothing to lose
REAL = Everything to lose (well at least all the cash on deposit)