If you trust the other guy to put one on his MIDI in, it's over-protection. But - in the real world, it's 10 cents well spent. Ground loops are a big problem in audio systems. MIDI is not particularly high speed, so there is no reason not to do it.
EDIT : as others have pointed out the opto in this schema has nothing to do with isolation. In fact it is not relevant to the MIDI interface at all as it is isolating between the driver and the internal circuit (this is the OUT side).
Take a look at this description of the MIDI interface hardware.
This version is done a bit differently - pin 5 is grounded, pin 4 driven high. It comes to the same thing. But the opto here is for interfacing with the micro, nothing to do with the MIDI wiring. (I didn't look hard enough the first time.)