It seems that with Studio One 5 it's no longer possible to disable the first output channel of an instrument in the instrument list. The checkbox for that is disabled. In Studio One 4 it was possible to uncheck the first output (though this setting was not saved AFAIR).
Why is disabling all outputs helpful?
I'm using Rayzoon's Jamstix drum instrument but only use it's MIDI output to trigger another Drum Instrument (SSD5, Addictive Drums etc.). It also has it's own drum sound engine built in, thus the outputs; but I'm not using it.
Unfortunately when using plugins with high latency on any channel, the plugin compensation doesn't work fine and the drum samples seem to be triggered with a delay. In SO4, when I disabled the default/1st output, compensation worked fine. My interpretation here is that when an ouput is present, latency compensation is also applied to the MIDI output, which is not desired, because the drums are triggered with a delay. But that's just a guess.