Presumably, a hell-bound man stands in the same relation to the to-be-saved vine and branches as a cancerous cell stands to the body. Damnation is the act of cutting off the wicked cell for the sake of the health of the whole spirit.
Now normally we'd say with St. Thomas that God "should allow evil to exist, and out of it produce good." (ST, I, 2, 3, reply 1) It seems to me, however, that the experience of cutting off a human being and throwing him into the oven is pure gratuitous evil, out of which no good whatsoever is obtained. Roughly, physical evil exists for the sake of moral good that arises out of fighting this evil. But damning a person is pure sorrow uncompensated or unbalanced by any good, physical or moral. The evil of burning away the spiritual cancer is in no way redeeming either for the condemned or for anyone else.
Even in regard to the body, cancer does only harm. No bodily good ever comes out of cancer. It's a destructive illness, as is also its treatment; e.g., chemotherapy consists in injecting poison into the body and hoping that survival is worth the price. Whatever does not kill the patient certainly does not make him stronger. A fortiori, the vine-and-branches are also weakened as a result of condemnations.
Finally, bodily happenings need not be perfect analogies to spiritual ones. Cancer may produce no physical good, but fighting and overcoming cancer may produce some moral good. But damnations are already on the level of the soul. Hence, no moral improvement happens to anyone at all. Hence, perdition is again shown to be entirely gratuitous.
For example, we stop loving a condemned person completely and forget about him. How can this experience possibly be of any value?
It may be objected that even if every act of damnation is gratuitously evil, nevertheless the rule or natural law of permitting people to be damned is good. But there is an easy way to preserve the benefits of the rule while escaping the costs, namely, by making suffering potentially rather than actually infinite. No matter how bad one becomes, there is always (though increasingly more arduous) way back toward the light. Even Jesus says, "You will not get out until you have paid the last penny" (Lk 12:59), suggesting that one's debt, no matter how great, is always finite and can eventually be repaid.