Bear with me here, my understanding of all this is still shaky. 
 
Can someone explain to me why a soft synth loaded in Sonar will only accept MIDI data if that synth's output is routed to an audio track? If I just load a synth and create an associated MIDI track, the synth will not register any MIDI data sent to it (I can see this in any synth that has a MIDI activity indicator, like Reaktor or FM8). It's only when I create an audio track and route its input to the synth that the MIDI data is heard by the synth. Not sure why I understand why this should be the case. 
 
I've also noticed that a synth with a MIDI output enabled won't output any MIDI unless it has an audio track routed to it. 
 
This is all a little annoying when you don't need that audio track, e.g. if using the synth's MIDI output to drive another synth. 

You need Audio output to hear the synth. Your MIDI activity is there if your controller is recognized and MIDI In/Out set correctly, but you will not hear it without audio to go in to Master. Not sure why you want to use one synth's output to drive another output. You can assign a MIDI track to any synth.

vladasyn
You need Audio output to hear the synth. Your MIDI activity is there if your controller is recognized and MIDI In/Out set correctly, but you will not hear it without audio to go in to Master. Not sure why you want to use one synth's output to drive another output. You can assign a MIDI track to any synth.

Ah I guess you have never come across the kind of synth that generates MIDI then. As an example, there are various Reaktor sequencer ensembles which generate a MIDI output that you can use to drive another synth. They don't actually produce sound themselves (unless you hook them up to another synth within Reaktor) so you have to connect them to a synth. In such a situation, you don't actually need audio output from the MIDI-output synth. You just need the MIDI. But you may want to route MIDI into this synth in order to automate the control of its interface. In such a situation, you apparently have to route a dummy audio track to the synth if you want it to accept incoming MIDI data as well. Of course you can just create that audio track and then hide it, but it still seems a little untidy to me. 

vladasyn
You need Audio output to hear the synth. Your MIDI activity is there if your controller is recognized and MIDI In/Out set correctly, but you will not hear it without audio to go in to Master. Not sure why you want to use one synth's output to drive another output. You can assign a MIDI track to any synth.

A number of reasons that I can think of.
 
#1. You use one midi track to play multiple synths in unison.
#2. You use VST midi plugins that only has midi in and midi out, and want that routed through a track with those on to next midi track.
 
You should be able to route midi out from a synth over to another synht according to help.
 
I never got that working - maybe there is something about this destination thingy.
I sent a project to support a couple of weeks ago - we'll see if they get around to handling that.

 I am with Member Sharke, I use jamstix and other MIDI generating plugs and I, too, am annoyed by the need for this useless ( useless to me, the program might need it badly) audio track. Seems to me that the point of the synth rack is you park your VSTis there and route the data out of them as you please. Of all the DAW software I know ( not lots , Sonar, Ableton, Reaper(a little)), Sonar has, by far, the most of these little quirks and limitations. I can't record the audio from a soft-synth, still, after all these versions? Mind you, Ableon Live has it's headaches too with MIDI generating synths.
 
 

worstcaseontario
Sonar has, by far, the most of these little quirks and limitations. I can't record the audio from a soft-synth, still, after all these versions?

Yes, and when you think you were smart and selected synth out from another audiotrack to be able to record output - track changes to a synth track - and there was the end of that idea.
 
Hopefully the remade routing for VST3 means that it might be around the corner.
 
It's not showstopper level for me - but would be nice to feel completely free to do anything that comes up as an idea.

Another quirk related to this is that if you're using a MIDI-out synth that has to start and stop with the Sonar transport, it won't get those transport play/stop messages unless again, it's connected to a dummy audio track. I agree, all of this defeats at least some of the purpose of having a synth rack. 
 
I'm in two minds whether to report this as a bug or a feature request. I wouldn't be surprised if the Bakers don't see this as a bug, but to me it's a bug because something doesn't work as it should without a workaround (inserting a completely useless dummy audio track).