Now, in class today MS raised an interesting ad hominem point against Sidgwick: if pluralism is unavoidable in any case, doesn't that undermine Sidgwick's earlier arguments? In other words, why does he settle on merely a 'dualism' of practical reason? If he can't ultimately decide between egoism and utilitarianism, then why can't common-sense intuitionism make a comeback? Or some kind of value-pluralist consequentialism? The permissibility of pluralism would seem to leave things wide open -- an even more "chaotic" moral order than Sidgwick, for all his pessimism, ever feared.
My response (on Sidgwick's behalf) is to propose that we distinguish pluralism within a single method of ethics, versus pluralism between the various self-contained 'methods'. Perhaps Sidgwick can consistently refuse to tolerate any hint of the former, even as he ends up committed to the latter.
PS points out that we may think this distinction is artificial. Rather than categorizing intuitionism as a self-contained method, we might just as well break it down into the 'gratitude' method, the 'justice' method, etc. I'm not sure about this though. There seems to be some greater aspect of unity here, insofar as the intuitionist thinks these various principles all contribute or speak to the common question of what one ought (according to common sense morality) to do. Principles of justice and gratitude do not seem to offer competing complete 'worldviews' in the way that (say) utilitarianism and egoism do. Put another way: the various intuitionistic principles call for mutual integration. But it's not as though we need to somehow balance the egoistic and utilitarian principles into one all-encompassing rational perspective (contra Roger Crisp). The whole point, for Sidgwick, is that they offer alternative rational frameworks.
MS (drawing on NB) suggests a different objection: perhaps even Sidgwick's preferred methods can be shown to admit of internal imprecision or indeterminacy (which brings the same need as pluralism for more authoritative adjudication). Given that personal identity is vague, Sidgwick would seem forced - by his own principles - to reject egoism (a concern for one's enduring self) as a fundamental method.
This might be just what Sidgwick needs. Assuming that impartial utilitarianism is subject to no such imprecision,* Sidgwick can avoid any kind of pluralism after all. For the only of his three 'methods of ethics' left standing is utilitarianism.
* It's not clear whether even utilitarianism can wholly avoid such objections though. NB floated the idea that it might be indeterminate what acts are available to us, posing trouble for the utilitarian principle ("perform that act, of those available to you, that will maximize utility"). MS suggests an example: you can hit the target, but you can't hit the bullseye. So start with the whole target and remove the fine-grained concentric circles one at a time -- somewhere in the middle will be borderline cases, whereby it is simply vague whether 'hitting the reduced target' is an available action you can intentionally perform.