Should diplomatic presence be part of the definition of a cruiser?
Short answerYes, but it is already implicit in the cruiser's principal task of long-range patrol and presence rather than being a separate defining feature.
NotesDiplomacy sits within long-range patrolling rather than forming a separate technical requirement. A cruiser maintaining presence can visit ports, show the flag, reassure partners, and make a state's reach visible. Those activities follow from endurance and independent operation even though Lord Hill-Norton's definition does not use the word diplomacy. A functional warship definition can contain political tasks without listing them one by one.