No extra hardware is required to "play" MIDI from a 8-bit computer, but a proper cable.
That is a bit missleading... the 128 K Speccy has RS232 support (and MIDI) built-in. So making a cable and accessing it from BASIC is easy, since already supported by the OS / Firmware / BASIC.
http://oldmachinery.blogspot.com/2012/11/spectrum-128-and-midi.html True, it is a software serial interface. "Bit banging" is used. And that can be used to implement RS232 on most old computers - PROVIDED they are fast enough to bit bang MIDI baud rate of 31250. That would be a stretch, say, for a ZX81. And CPU load is very high with that method.
Virtual Net 96 for the CPC also uses "bit banging", resulting in high CPU load:
http://www.cpcwiki.eu/index.php/Virtual_Net_96In addition, MIDI OUT is easy to implement, even from BASIC once you have fast serial interface API in ASM (be it software serial interface via Bit Banging or real UART hardware).
MIDI IN is much more difficult. I don't think we will see MIDI IN being implemented on the Speccy in that way.
For that, real RS232 / MIDI hardware and ASM to process the real time MIDI messages is required.
So, saying that "only a proper cable is required" to play (= MIDI OUT) on "8-bit home computer" is a bit of an oversimplification. Certainly, as always - it depends. Is the the CPU fast enough to do bit banging at 31250 bauds? Do I have a fast ASM implementation of a serial interface (certainly, you cannot bit bang from BASIC!), etc.