I appreciated your idea earlier and practiced it in my real trading.There is one risk involve in it that i experienced practically.To invest in a long term basis you should have a good amount of money that will ensure that you will not be a victim of margin call.Mostly we don't have much money from bonus.The amount which we get from bonus is perfect for scalping but not for long term trading.You should go for a long term trading when you have a good amount of money.I was a victim of margin call in gold trading and lost all my bonus.Metal like gold is very volatile and very risky for small account.So be careful before doing that.