Grumble...
Does anyone actually use this? If so, for what?
Why is it enabled by default?
I can't for the life of me figure out why anyone would want to turn it on, especially since you can't tell it where to go.

Yes
 
To transfer MIDI data from the plug-in in the synth rack to the input of another MIDI or instrument track.
 
The default was changed in X3, prior to X3 the default was disabled by default. There is some question whether this should be a persistent setting in the "Insert Soft Synth Options" dialog.
 
The data goes no where unless an instrument or MIDI track is set up to listen to it.

scook
Yes
 
To transfer MIDI data from the plug-in in the synth rack to the input of another MIDI or instrument track.
 
The default was changed in X3, prior to X3 the default was disabled by default. There is some question whether this should be a persistent setting in the "Insert Soft Synth Options" dialog.
 
The data goes no where unless an instrument or MIDI track is set up to listen to it.

WTF is it enabled by default? If I want to use it, I'll turn it on. It goes to any selected midi track that is set to omni which is the default. That's pretty random.
Nevermind, I'll just assume that you know everything and add the extra step to my workflow to turn the fricking thing OFF. What a PITA.
FEH!
 

Enabled by default since X3. It can be set when inserting a synth using the "Insert Soft Synth Options" dialog.
 
Maybe turning off "Always Echo Current MIDI Track" in Preferences > MIDI > Playback and Recording will help.

scook
Enabled by default since X3.
 
Maybe turning off "Always Echo Current MIDI Track" in Preferences > MIDI > Playback and Recording will help.

Thank you sir, much better.

To test Midi Output, I've tried adding instrument tracks for z3ta, rapture pro, addictive drums, and session drummer and made sure I checked Enable Midi Out for each.  The only one that seems to take the setting is session drummer.  For each of them, if I look at the VST2 or VST3 drop downs in the VST headers, session drummer is the only one that has Enable Midi Output checked.  If I create a separate Midi track and see what my available inputs are, session drummer is the only one that shows up.  When I try to have Session Drummer's Midi Out play Addictive Drums (by setting AD Midi Input to Session Drummers Midi Output), I get nothing happening.  I thought it should play the AD drumset when I play a Session Drummer pattern.  Why would this not be working?
 
Also, I thought it was odd that I cannot check or uncheck Enable Midi Input or Enable Midi Output in the VST header for any of them - they're all greyed out and unchangeable.