Having to "mix correctly with the other instruments and what you hear directly from the same instrument" seems like one of the (few) cases you can't account for with just intentional delays and buffering, I agree with that.
But most of what I've seen on the topic seems to be complaining about other problems. The JACK FAQ says "JACK requires real-time scheduling privileges for reliable, dropout-free operation". I guess maybe that's just because they set their buffers so small to try to reduce latency. For users who aren't interested in the use case you described, it would be much easier to just increase the buffers and allow 50ms latency rather than have to go through all this trouble to try to get the kernel to do realtime.