I agree that it's silly to keep more than one needs on any exchange at this point in time.  That goes double for MtGox by virtue of their reputation.

But I disagree that it is 'entirely' the guy's fault.  If he used good practices as he described then it was almost certainly yet another internal breach or system error.  That is fairly unacceptable.  I must trust my exchange with several k$ at a time if I want to do the type of business I am interested in.

A site which has been breached and/or is careless in one way or another could lose that money in 1 microsecond and that is completely unacceptable.  I choose my exchange(s) as best I could with this in mind, and I would consider myself partially to blame in the event of a loss by virtue of not choosing carefully enough, and partially my fault for being involved in Bitcoin at all, but certain not 'entirely'.