it depends of what you need. If there are some object that will be frequently reused. I think it is better to keep it in memory instead of having to reload it (which is a very CPU consuming process if your object is complex).
I never tested the following proposition but i was thinking about it... Is the cost of cloning an object in term of CPU similar to the fact of creating a new one using the Loader class? I think cloning should be far more efficient but i never tested it. If so, you could create a set of frequently created objects (those object would not be linked to the world and would only be used to create cloned models that could be linked to the world). The CPU usage would be reduced but it would cost you some space in memory. Everything is a question of balance
I suppose if your Object3D is created without being linked to anything and you do not use it for a certain period, the garbage collector should destroy it... But maybe there is some kind of list in the JPCT architecture where all objects are referenced once they are created (which could explain the NO_OBJECT field)... I suppose only Egon can answer to this question.