Carla (and maybe Cabbage?) uses it for control messages but there's the problem that it only works if you've got a big single host running a lot of subject plugins; the idea was supposed to be that your sequencer would send midi to your sampler and your tracker would send control messages to your sampler over OSC and all three of these would be peer programs. There's not a particular technical reason it doesn't work it's just that very few programmers built software to take advantage of it.