You bring up a good point, but my experience is that knowledge of the project is a poor proxy for importance. I have about ten Areas of Focus. I could not rank order them in importance, although I could perhaps group them. If I look at a project name/title, I pretty much know what area it belongs to, although there is a bit of ambiguity for some projects. However, I don’t know the relative priority of a project just by looking at it. When I look at a next action for a given context, I usually have a reasonable idea of time and energy required. These are traditional GTD criteria for what to do next, with priority coming after them. But determining the priority of a next action by knowing its project and considering the project‘s priority is a lot to ask in making a quick decision. Of course, there are projects and next actions which do have clear priority because of importance and/or timeliness. Those do surface in the weekly review, but they give rise a relatively small fraction of next actions.
Ah--this may partly come down to the number of active projects.
The number of active projects that I'm comfortable with is REALLY REALLY LOW. I long ago accepted the, for me, relatively minor opportunity cost ("You just ordered from Gardens R Us. You COULD have ordered stuff for next year's projects too!") inherent in hiding most of what I will do in the future.
So if a project is present in my active lists, I'm very familiar with the project and its relative priority.
One could, in fact, argue that my number of projects is so low that priority shouldn't be a factor, because the very presence of the project in the active lists means it's a fairly high priority--most of my prioritization comes in whether a thought has even been promoted to a project, and then whether that project has remained active or been demoted back to Someday/Maybe.
But I do pull projects into the active lists just because I want to do them--a thought gets promoted and stays active, but it's really not that high a priority. Out of the dozens of totally unnecessary flowers I'd like to grow, I choose THAT one and make it a project. But if it ends up in opposition to snap beans, it loses.
So the identity of the project is a major indicator of priority, in that scenario.
I like having next actions attached to projects too, but I think there is a tendency to exaggerate its value and ignore its costs.
And the tool may be a major factor here, too. I would have to actively work (a cost) to UN link actions and projects.