It's rude, and it demonstrably increases traffic.
It's an arbitrary game, and everyone makes up their own "rules of decency" and gets mad when someone follows the rules of the road rather than the rules of their head. Yes, people "know" about choke points, but that doesn't mean it's wrong (or rude) to use open lanes. So long as those lanes are open, they are going to be used, so everyone may as well make use of the space.
Is it my fault that others choose not to use those lanes? No. Is it your fault that it upsets you? Yes. It's not rude. I'm not being impolite. I wish everyone would just follow the same set of rules so we could stop being mad at each other. Which set of rules should we follow? I vote against the ones that change in with the person you're asking.
Having said that, I don't really like when people change lanes just to jump ahead of one or two cars. It doesn't upset me and I think it's fair game so long as it's done safely, but I empathize with the distaste for that a little more.
This is not only rude, but also blocks the other(s) lane(s) bringing traffic to where it wasn't before.
Its almost like say it's ok to pass in front of other people in the theater line.
The lane is open until it is not, and as long as both lanes are open they are both "right" (in the non-directional sense, obviously). Why rely on clearly nebulous (sometimes variable, sometimes non-existent) social norms rather than concrete traffic norms? Use the open lane, or don't complain when others do. It's not asshole behavior, it's common sense driving.
There is no reason to take offense while driving so long as people 1) follow the rules of the road and 2) do not endanger others with reckless driving. Using open lanes does neither, and it only inconveniences the people that refuse to use the roads as they are intended to be used.