Why use Linux for that though? Why not build the machine like a 3D printer, with a dedicated microcontroller that doesn't even run an OS and has completely predicable timing, and a separate non-RT Linux system for the GUI?
One thing to keep in mind for people looking at the RT patches and thinking about things like this: these patches allow you to do RT processing on Linux, but they don't make some of the complexity go away. In the Klipper case, for example, writing to the GPIOs that actually send the signals to the steppers motors in Linux is relatively complex. You're usually making a write() syscall that's going through the VFS layer etc. to finally get to the actual pin register. On a microcontroller you can write directly to the pin register and know exactly how many clock cycles that operation is going to take.
I've seen embedded Linux code that actually opened /dev/mem and did the same thing, writing directly to GPIO registers... and that is horrifying :)
More effort can be devoted to microsecond-level concerns if the microprocessor can have a 1ms buffer of instructions reliably provided by the computer, vs if it has to be prepared to be on its own for hundreds of ms.
http://linuxcnc.org/
Marco Reps has some entertaining and informative videos on LinuxCNC with EtherCAT
The difference is more that you can control those output lines with really low latency and guaranteed timing. USB has a protocol layer that is less deterministic. So if you need to generate a step signal for a stepper motor e.g. you can bit bang it a lot more accurately through a direct parallel port than a USB to parallel adapter (which is really designed for printing through USB and has very different set of requirements).
But alas, it was decades ago, so it's possible I'm wrong ;)
This is the closest reference I can find: https://www.sfu.ca/phys/430/datasheets/parport.html
The card does have an interrupt but only the ACK signal can interrupt. Not the Data lines. ACK makes sense since it would be part of the printing protocol, you'd send another byte each interrupt.
On a PC you have millions of lines of kernel code, BIOS/EFI code, firmware, etc. You have complex video card drivers, complex storage devices. You have the SMM that yanks control away from the OS whenever it pleases.
The idea of running a dangerous machine controlled by that mess is frankly scary.