Huh? I wasn't making an argument, I was paraphrasing the court's majority opinion, or at least your presentation of their reasoning. For the record, I agree most gun control legislation is pointless for precisely that reasoning, that there are already so many guns out there that adding purchasing restrictions isn't going to have any real impact.
That's a good sound bite, but it isn't really true. People's votes are readily manipulated through the science of marketing (i.e., propaganda). The most effective marketing comes from those with deep pockets, people who can hire the marketing, production, and distribution expertise to push the buttons that manipulate the public into supporting candidates and agendas. That is one way that money corrupts democracy, by giving an elite few undue influence over voters.
Limiting aggregate spending somewhat limits the number of politicians one can buy. If special interests can give $2600 to every member of Congress, they have a much easier path to getting their legislation passed. That said, I agree there enough other ways to indirectly bribe Congressmen that the aggregate spending limit was probably only a minor hurdle.