Good question, I think there are many different approach on solving running multiple services. So it boils down to what you need, and what works for you. One advantage might be the ease of use. Using Porter you don't need to know anything about Supervisor, systemd or other more technical implementations. Porter abstracts away these things so you only have to write a simple .yml file.
> Using Porter you don't need to know anything about Supervisor, systemd [...]
Using systemd you don't need to know anything about Porter.
> One advantage might be the ease of use. [...] Porter abstracts away these things so you only have to write a simple .yml file.
Is the systemd unit file really that much more complex? The example `mpd` service file might take more lines than the equivalent Porter .yml snippet, but the DOS/Windows 3-era ".ini" file format isn't exactly rocket surgery, and there's only one kinda weird bit of boilerplate ("WantedBy=default.target") that isn't immediately obvious why it's there. Other than that, the systemd config for a simple service is, well, simple. (At least, it looks that way to me.)
Yeah, there's a lot of complexity you can add with systemd units, but only if you have complex use-cases that need it. As far as I can tell, the only alternative to "Yeah, there's a config option for that, but it can take a bit of reading to find out its name" is "No, you can't do that, sorry".
As Alan Kay's classic adage goes, "simple things should be simple, complex things should be possible" :-)