Yes, this. The equivalent in the physical world is scribbling ‘Reply before Thursday’ on a letter in red ink; or maybe using a post-it. This is fine for some letters/documents, but what if your task is to make a copy of your birth certificate? Are you going to scribble on it, or leave it lying around in plain sight? The whole point of a todo list is that it’s a list, an index you can carry around and manipulate, while the things that need working on are safely filed away.
(and yes, this contradicts my post-it analogy elsewhere. It’s so elegant, I still half-believe it has to work for that reason alone, but I guess beauty only means truth in math and physics, not in such a chaotic system as human task management).
A task management system is one place that really benefits from tag-like metadata, and the JD system IDs are really useful ones. I have some issues with the todo.txt format, but it definitely works well. Using a JD ID as a context or project or other property value (or ‘group’, as you demonstrated with Things) is very effective.