Sorry, but i dont completely agree with this.
It depends greatly on what the amp is.
We run bass through guitar amps all the time where i work (Recording studio/ rehearsal rooms) and find more damage to the bass amps to be honest!
But we are generally using Half stacks, most 4x12s can handle anything you throw at them.
However, some "small" guitar amps (and bass amps, come to mention it) can hardly handle bass at all, sod all headroom, and tiny speaker mean really bad bass sounds, much like the morning after a bad curry
Matt