Overall I’d say it’s probably done more good than harm.
I think the one thing east asian states tend to have in spades is pragmatism. The US, on the other hand, is very often engaging in the region for ideological reasons. For that reason, it has tended to engage in bloody, somewhat pointless wars that have often span out into civil wars in neighboring states (Cambodia, for example) or produced traumatized, basketcase states (North Korea, for example, or pre-revolution South Korea).
The country-years of war in the region between 1950 and 2010 could potentially have been in the order of 1000s, but has been in the low 100s.
I'm not saying US threat of war did actually prevent a lot of smaller wars in the region to occur.
I'm also not saying if it did, that it was a good thing. (Maybe like with forest fires, many small wars are better than a few big ones.)
But that's the argument used by people favouring this US threat of war.
It's more the promise of security. War comes from insecurity (or for a few among non-democratic countries, the desire to take something from someone). The US security guarantee for Japan, S Korea, etc. meant that they could act with security.
If that's a contribution to international peace, I hate to imagine what you'd think was a contribution to regional instability.
Even on a modern level, I think it's arguable that the 90's gulf war, and then the 2000's one, were the drivers behind a massive up-arming amongst states around the world that saw themselves as on the US's potential 'regime change' hitlist. It's often touted as a reason for China's massive arms buildup, for example.