V3 has some utterly/unnecessarily complicated shite going on, like offscreen documents, and I think it will get worse as time progresses. Google really needs some bludgeoning.
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The counter argument is that ad-blocking is an arms-race and the only reason a subset of traffic blocking methods is going to work at all is because there isn't a big enough incentive yet.
Other than that the arguments in favour of V3 are going to be about the same as for any API update. My guess is a few extra APIs and more granular permissions.