I admit that most modest (and thus politically achievable) voting reforms would be unlikely to result in immediate wins for small parties, but that sort of stab

I admit that most modest (and thus politically achievable) voting reforms would be unlikely to result in immediate wins for small parties, but that sort of stability is probably a desirable property of any electoral system. The main benefit in the short term would likely be that large parties (which are themselves coalitions of various factions, with internal dynamics that play out behind the scenes and usually without popular support) would split into smaller, more representative units. For example, if the Democrats split into a Hillary party and a Bernie party, the Bernie party might never have one of their members become the Speaker of the House, and two parties would formally or informally act as a coalition, introducing and passing pretty much the same bills as the (combined) Democrats do today, at least initially. Rather than advocates complaining that "nothing actually changed", though, I think they'd notice that the supporters of these new separate parties were reinvigorated, enjoying the ability to have a party that truly speaks for them and doesn't have to compromise at the stage of choosing its leader and identity. These new parties would allow the political discourse to change, and shift the Overton window, while also allowing parties to appeal to non-voters who had previously been faced with a choice of two evils, or uncompetitive districts.