i have a theory, maybe its nothing more than that, but to me it sounds reasonable...

dictionary definition of utility:
The quality or condition of being useful; usefulness;
and:
Designed for various often heavy-duty practical uses. e.g. a utility vehicle.
(this was the closest i could find of the term utility, and its application to vehicles)

When ford first invented the ute, it was a very useful car. (the idea was mentioned before of people being able to use it for work during the week, and still having a car to drive on sunday) Naturally they called it a utility, in accordance with the definition above. Later, the trayback version was invented/used, and in order to show there was a difference between the various types, they named it a trayback. The fact still remains, that it is a useful vehicle, and so under the dictionary definition, it is possible to refer to a trayback as a utility.

We could, using this idea, probably apply the term "utility" to every type of vehicle, as they are all useful. The original application of the term ute was to one with an open load space, the type we are discussing, and so it has remained specific to this category. as a result, individuals apply the term "ute" or "utility" to this group, or to a certain variety in this group.

Since there is no dictionary definition of the term utility, and its application in this area (at least, not that i could find, and yes dodgeman i did research ) the term can be applied as each individual wants to...

so we can probably all continue arguing/discussing this until our fingers are so sore we can't type anymore, and we still won't get anywhere.
maybe we should respect each others definition of "ute" and leave it at that.

interested to hear your views on this...