A solo musician or duo will often want to improve the sound of their performance by adding the sounds of drums and bass together with possibly backing vocals, strings etc. This is particularly true of the club/Pub entertainer playing covers (i.e. their own version of well known songs). In the old days this would have been done either by recording onto DAT or cassette from studio recorded tracks or by using a sequencer and synthesiser live. Both of these options were troublesome - pre-recorded tracks would be expensive and the tape format a nightmare for anyone needing to jump to the right track when a request from the audience comes in. The synthesiser option is also a nightmare for the average technophobic artist - being expensive, difficult to set up and with a number of things to go wrong especially on the road.
The minidisc is perfect in this situation. Assuming you record the backings in mono (many PA systems / amplifiers are only mono anyway) you get 140 minutes of backing which means one can often get away without making a disc change for many gigs. The title of the song is clearly visible even in the dark, and an artist can simply scan through the songs on his disc to decide what to play next. The audience is never left waiting for long.
One feature which is particularly useful here is 'Play and Pause' for single tracks - which is available on many Sony models but sadly not a lot of portables. Also backlit screens for portables would help.
The Sound quality goes without saying. I actually have a synth, but prefer the convenience of MD rather than setting up Synths and associated equipment.
I have also noticed a number of companies who offer midi files (for Synths) now offer the tracks on pre-recorded MD.