Nice idea, however it would be lot easier and faster for my to draw the digram than it would take to write the code in strange language.
Maintainability. With declarative text-based diagramming, you can check these in as text files, and when the system evolves or the diagram has a need to change, it's easy to make a change and re-generate the image.
Once you learn the rules for the type of diagram you're making, it's much simpler than opening up <diagram tool of choice> and trying to shuffle the pieces around to make room for the change, or re-sketching the whole diagram from scratch on a piece of paper.
See: