Suppose I were totally indifferent about the size of government. Because I thought that the government was always equally good (or equally bad) at spending money as private households and firms. And not just at the margin, but everywhere. Never better, never worse, but always exactly the same. Whether G is 1% of GDP, or 99% of GDP, I don't care. The marginal benefits to government spending (adjusting for the marginal deadweight cost of taxes, and relative to the marginal benefits of private spending) did not diminish.
Then I would find it hard to argue that fiscal policy was worse than monetary policy. You fiscalists want big government spending sometimes, and small government spending at other times? Whatever. We could use monetary policy instead, but who cares?