Good discussion. I too have had long standing problems with mute and solo with multiple midi tracks going to a VSTi which has multiple outputs. The present implementation is certainly irritating and unhelpful as said above. Ctl-Click has a different wild action that is hard to imagine useful for anyone (all outputs get soloed when you UN solo a midi track with control click).
So improvements welcome. It is extremely common to want to mute or solo ONE midi track out of a group of midi tracks that go to the same VSTi and to the same output, in a situation where a different group of midi tracks also go to the one VSTi but to a different output (and this other group should, ideally, not be affected at all by the mute or solo).