Well, I guess the pro of Garak’s idea that is ultimately supported and carried out by Sisko is that it worked. The deception achieved its desired effect. An alliance was borne, and a war was won. By committing these actions, while reprehensible as they might be, Sisko achieved his goal of upholding the laws of the Federation and protecting all life in the Alpha Quadrant.
During the course of this episode alone, the Dominion successfully conquered Betazed, placing Dominion fleets in a prime strategic position to attack at least four more key planets, and thanks to Sisko’s actions, these attacks would never come to pass.
Also, as an afterthought, had Sisko not acted in the way he did, it is highly likely the Romulans could have forged an alliance with the Dominion, already having a non-aggression pact with them, as Garak correctly pointed out.
While Sisko’s ultimate goal was achieved and a war was won by the “good guys” at what cost exactly was this accomplished? On a somewhat small scale, Sisko had to participate in several nefarious acts, including but not limited to, falsifying official station reports, bribing one man to save another’s life, intimidation of his chief medical officer, being an accessory to not one but two murders carried out by Garak in the name of their cause, and, of course, a complete disregard of the Prime Directive when luring the Romulans into an alliance under false pretenses, and that is only on the smaller scale.
The war would be won, and Sisko would never have to post those dreaded casualty lists again, but the Romulan government would have their own casualty lists to post. The Romulans would see massive casualties, in both soldiers and civilians alike, as their payment from the Federation for joining a war that was never theirs to begin with.