Just because vices sometimes produce good results doesn't mean we should accept them as normal. Fevers can help cure certain diseases, but it's still better to avoid fevers entirely. It's a terrible way to get healthy — needing sickness to make you well. The same goes for anger. Like poison, falling off a cliff, or getting shipwrecked, anger might accidentally help sometimes. But that doesn't make it healthy. After all, poisons have often been good medicine too.
In the next place, vices ought not to be received into common use because on some occasions they have effected somewhat: for so also fevers are good for certain kinds of ill-health, but nevertheless it is better to be altogether free from them: it is a hateful mode of cure to owe one’s health to disease. Similarly, although anger, like poison, or falling headlong, or being shipwrecked, may have unexpectedly done good, yet it ought not on that account to be classed as wholesome, for poisons have often proved good for the health.