i think i find little use in them because i tend to focus my trade into science instead of gold. i find little use for them as i cant steal techs. i have no interest in the internal workings of an enemy city, and usually not interested in buying a weak unit.
if i focused a game into making gold and stealing tech, iscience would probably be popping out diplomats all the time as the units being purchased would be the same level as the ones i can make and stealing the tech would be a secondary huge use for them