In all his solo career he has never had any act open for him. He seems to be set in his ways in these kinds of things. A lot of people use the earplug monitor system and I use one as well and if setup right really does enhance your vocal performance. He must of tried it and didn't like the thing stuck in your ear.
You can't sing out of tune cause your hear the perfect mix in your ear. He could have benefited with this monitor system in the confines of the Caver gig back in '99.