I never bothered to add it up. That's a cool statistic.
When exchange is compromised then you start worrying about it like NXT community was with Bter fiasco.
Hacker took 10% and then suddenly many wanted rollback or block coins, and people have concerns about easy attack on network.
They wanted rollback and shit like that but bter owner payed hacker some money and problem was solved.
Till real problems this is fun statistic later such coin is really hard to promote.
Just because this is not happening often it doesn't mean its kind of serious problem.