Looking back, the 8-bits struggled to be truly useful, though the c64 succeeded more than most. But I would say that MIDI sequencing was an area in which the 8-bits could be eternally useful, despite the constricted system resources.
I used to reckon that a computer wasn't a computer until it had a full utility cartridge you could master, in my case the Action Replay. Other people may say their systems weren't complete until they had maybe a modem or a second disk drive.
Nowadays I think a Commodore only has the full shilling when it's loaded down with disk drives, Centronics printer interface, utility cart', image scanner, PC serial networking, Sound Expander and keyboards, various knobs and control units (inc. mouse and paddles), as well as the essential MIDI cartridges. MSSIAH; oh yes.
I don't know about you but I've never seen a MIDI program from the 64's heyday which directly fires the SID as you input MIDI in realtime. Closest to the mark is the so-called classic Advanced Music System. (I say so-called because I've never been able to get to grips with the very impressive looking-program because I can't find a manual in any form anywhere.)
Great 80s home computer music-makers:
http://www.youtube.com/watch?v=cLfYT6yXgEcStill, the SFX Sound Expander could be fired by MIDI by 1987 (though you could have used its outboard keyboard since 1985 when it was released), plus there still being a wowing array of software to pick from anywayin those days anyway: