(1) First, it impossible to win every conflict without battle because the other side might be eager to fight. You cannot stop an irrational enemy determined to attack.
(2) Second, you might win without fighting today, but that doesn't guarantee the root problem of the conflict won't flare up again tomorrow. In other words, the peace is only temporary and that battle is inevitable. Wars are a part of civilization.
Let me address both arguments.
(1) First, although it is impossible to avoid fighting every enemy, that doesn't mean you don't strive for an ideal goal every time. Sun Tzu didn't say it is easy. In fact, he said winning without fighting requires the highest skill. Trying to save lives and stop suffering are probably worth the effort. So once you decide to make that effort, what would incite you to fight before now isn't so automatic. You now would want to step back and figure out a way to resolve the problem.
Furthermore, you are more likely to prevent the problem from occurring in the first place. Prevention is very plausible: preventing problems when they are small and manageable has a higher success rate than when they are full-blown crises. And even if the other side is eager to fight, that doesn't mean you can't draw him away by taking what he values elsewhere. That is, instead of simply engaging the attack head on, you are using strategy to encourage him to go to where you want him to go.
(2) Second, yes, winning without fighting doesn't necessarily mean that anger won't flare up in the future. But this isn't a bad thing for Sun Tzu. The present is important to him. Would you rather take the path to peace today or have the possibility of opening up a path to peace tomorrow? One day without fighting is one day fewer than if you did nothing different.
Furthermore, not fighting de-escalates anger and resentment. Let there be a longer cooling period so there could be a higher chance that lasting peace takes root. After all, Sun Tzu did say that with time, anger can revert back to joy but the dead cannot be brought back to life. So why even say words of anger in one situation when those words cannot be forgotten for a lifetime? Why make a permanent decision over a temporary condition? One should make the effort to employ discipline and self control. Achieving a strong sense of self respect and self worth would prove even more valuable than the positive outcome itself.
In summary, what seems to be an idealistic concept from Sun Tzu is really quite pragmatic and smart. Ultimately, our lives -- and the lives of those around us -- would benefit immeasurably if we use this powerful lesson to our advantage at work and at home.