One reasonably powerful wizard defector to the Muggle side could enlist other wizards via subterfuge or magical control, and such wizards could create magical items to aid the Muggles. If the Muggles were sure that they could trust such a defector, they could hand em a nuke and have em wipe Hogsmeade off the map in spite of all its anti-Muggle protections.

There are plenty of Muggle-borns, half-bloods, wizards married to Muggles, and wizards merely fond of Muggles a la Arthur Weasley, that in an all-out war, there would be considerable numbers of potential defectors.

By contrast, a Muggle defector to the wizarding side would be harder to come by per capita, and also less useful, as Muggles don't have special abilities that let them be particularly useful to wizards.