Good sustain would also be useful if you're covering a line that was originally played on a synth. I remember having to play a Massive attack song at uni that had whole notes in each bar. I played it on fretless. Birdland is another that springs to mind, and another I had to play at uni, (not surprising my anxiety problem resurfaced then ). Not songs that you get asked to play every day, but if you hit an open E in a song expecting it to ring out and it dies after half a bar?