ATM if you try starting JACK on a device currently in use you will be fed a most unhelpful error like:
the playback device "hw:0,0" is already in use. Please stop the application using it and run JACK again
This is presuming that the user knows what app it is that is using the device and what the device name is or at the very least how to find out both these bits of info, which only seasoned NIXrs will stand a chance of doing. I don't think I'm exaggerating when I say that thousands of people have given up with JACK at this point even before they've given it a proper chance, when all it would take is for JACK to say:
"Process foo is blocking JACK from using device bar. If you want to terminate process foo type 'killall foo'"
or something to that effect.
Adding this one feature will do wonders for JACKs usability and acceptance IMO.