This is interesting, though I wonder if the wider audience of asciidoc users will find this as an odd way to interact with the engine.
I had originally thought you'd call guard from asciidoc to have it watch the file it was called to process. Different way of doing things, not saying one approach is better than the other though.
I think we want to support both cases. There are going to be people that prefer one way over the other, and that's fine.
What I like about the Guard plugin is that it gives advanced users lots of flexibility to do exactly what they want with the watching. I suspect that the direct integration into the asciidoctor command will be more limited in scope (since the goal of that integration is to require no additional setup).
Principal Software Engineer, Red Hat | Author of Seam in Action Registered Linux User #231597