Setting *basedir* might be useful in case you have a directory structure with one master document including other documents from subdirectories. Paths in included documents might be relative to the master document. When editing one of the included documents, it is useful to set *basedir* to the directory of the master document to make includes work. This is also related to *{indir}* and *{infile}* attributes, currently disabled in asciidoctor.

Of course you are right, it would be best if the user would not need to worry about that.