It also has the advantage that, if later this solution needs to grow past a simple command line tool, I can just grab the Swift code from the script, put it in an Xcode project, and make a Mac app out of it.
And in the domains where shell is a good choice, it's a far far better choice than Swift. Maybe that will change, but I doubt it.
I've implemented a couple of medium-small tools over the last two years and as an exercise did them in both shell and Swift. The shell versions are simpler, more maintainable, and more robust than the Swift versions. Swift's verbose "correctness" is not an asset in the domain(s) where scripting is useful. Gluing programs together is one. Frankly, text processing is another. An invocation of sed with a simple regex becomes a monstrosity when translated to Swift. The less code you write, the less you have to maintain.
This is because Swift’s regex support is overly verbose, being lifted straight from Objective-C. It’s not an inherent problem when the language.