The main reason people say holding your money in an online exchange is because you have to be certain that your money is safe with that exchange. This is likely impossible to guarantee 100%. Take for example this situation. Let's say you give $100 to your friend James because he can hold onto it. While he has the 100 dollar bill, you don't know what he's doing with it. You don't know if he's going to get robbed because he has a 100$ bill or if he's going to just turn against you and take your money. That's the danger in putting your trust into other online exchanges -- there are always "if"s.