I use some VSTis to control hardware synths. And those VSTis don't work (don't process midi and automation) if there is no audio track to which their output is connected. In result I have dozens of tracks doing nothing but only enabling VSTis (I use one VSTi instance for each sound to store presets and automate). Yes, I can hide these tracks, but it would be much more convenient if it could work without them.

Can you use instrument tracks for those uses?

I don't need midi track either. I send midi directly to hardware synth, and get audio from it. I can place controlling VSTi's automation on any track, and I need it working. And Midi Output.
There is wrong logic. VSTi is not only about audio, there is midi and vst automation also. Audi In/Out, Midi In/Out, Automation In/Out. All this can be used separately. So why if i have no Audio Out, I can't use Midi and Automation? Maybe there is some reason for it, or it works like that by tradition, I don't know.