A lot of things happen when money is on the line. Suppose that your testing tells you that you have a 60% change of hitting a profit of 4 points versus a loss of 2 points. Pretty good odds, right? And suppose that you've simmed this for quite some time.
But suppose that when you start trading real money, you find that after your first losing trade, you get into a winner, but don't hold it for 4 points, you just get out at a profit of 2 points because at least you've back to break even. Well what you've done now is completely changed that statistical nature of your trade methods that should make you be profitable.
When money is on the line, everything changes. I've read that the pain of a loss doesn't equal how much pleasure you get from a win. When you're SIM trading, its all just points, but when you're trading live, its actually money that goes up and down. Now should it be this way? Of course not. But if you see that when trading live, you have trouble holding onto a slight winner because you're scared of it going against you, then lots of changes will need to be made.
So I think live trading needs to be a component of SIM trading and learning how to trade. Its only when trading live that you learn where the real problems are.