Is the ground necessary with an opto-isolated RS422 signal
midi is not like rs422.

In a midi input the incoming signals go straight to a LED. There is no other circuitry on the isolated side of a midi input (midi outputs are usually not isolated). 

RS-422 uses differential signaling, but it feeds that into a bidirectional electronic transceiver, which has a limited tolerance for common mode voltage difference. So the "signal ground" of the RS-422 transceivers (which may be separate from the signal ground of the rest of the system, which in turn may or may not be separate from mains ground) need to be connected together.