Hi Faré,
Probably, I'm the first one who started the custom.
However, I suppose it's technically not widely used because of Quickdocs,
since it also searchs README file at the project root.
Perhaps, it may be because CL-Project, my project generator, has been adding the code to the new ASD file.
I just thought it might be useful if it could be refer by calling 'asdf:system-long-description' from REPL,
or someone would make some utility tool using long-description in the future.
To be honest, I'm not a big fan of it anymore and I don't follow it in my recent products.
I'm willing to delete the code from CL-Project, though I have nothing I can do for existing projects.