An excellent Washington Post op-ed by Paul Saunders makes the key point against fashionable schemes for a Concert of Democracies: "Moreover, trying to create a 'Concert of Democracies' inevitably invites a 'Concert of Non-Democracies,' which could be very damaging to American interests and values." Indeed, I would say that it would not just invite but in many ways force the leading non-members to form an alternative club. The nominal rationale for doing this is that autocratic UN members can block humanitarian action, but as Saunders writes:
Nor would the world be safer for democracy. In fact, it would be far harder to promote economic development, political change or human rights in an increasingly divided and unstable world. The great global advance of democracy occurred during the relative peace and prosperity after the end of the Cold War -- not during the struggle between the U.S. and Soviet blocs.
Exactly. It's wildly underappreciated, but far and away the best thing we can do for the spread of democracy and human rights around the world is to do what we can to avoid a return to a situation where developing countries were perennially finding themselves playing the role of staging-ground for superpower proxy wars. For the US and China, or the US and Russia to shift from the current mode of wary peace to actual hostility would be a fiasco much, much greater than any sin of omission that might be caused by deference to international law.