The Mt.Gox debacle affected a lot of users and I think almost everyone who was using Bitcoin at that time had deposited something there. It's not like the most trusted and popular Bitcoin exchanges nowadays are not susceptible to hacking incidents because anything could happen ranging from inside jobs to legitimate security breaches. And also if your current exchange hasn't had any problems till now that doesn't mean it won't have in the future so only hold amounts that you can afford to lose.