What made you go with that rather than trying to pick a sensible default like the number of cores available? Or was it just a general goal to require explicitness?
It was simplicity, of design, implementation, use, and indeed explanation. There's only one default, the default if the --jobs option is not used; rather than there being two defaults, one for not having the option and another for having the option but not the option argument.
More generally, I try to avoid optional arguments to command options, in line with the guideline in the Single Unix Specification. (I actually picked such ideas up many years ago, before the first POSIX specification, from a book by Eric Foxley titled Unix for Super Users, where there was an appendix on command line option syntax.)
Of course, if the user wants to use the number of cores available, then xe can work that number out and pass it as an argument; and indeed I have done that very thing in some of the package/make scripts that I have published.
