Single continuous mesh vs disconnected parts Sometimes I am modeling something that has parts which would be much easier to attach separately, as opposed to joining them. For example, a sword. I could model the blade and hilt and hand guard separately, or I could do it such that it is a single, joined mesh. 

I understand that if parts are going to be animated like a wheel on an axle it obviously can make sense to use separate parts; and that a head should probably be connected to a neck for smoother lighting and stretching. But otherwise, is there any reason not to use separate parts if it makes the process easier? Does an engine work harder, or does it use more texture memory, etc.?
The best reason to use separate parts is because it makes the process easier. And because it is much easier to join two objects into one than to split one object into two, in my own workflow, if I have the slightest hint of a doubt that I might want to be able to access the items separately later on, I make them separate, 

But separate does not always mean two objects. Sometimes I make two or more meshes as sub-objects of the same object, for example, I might make the rear wheels of a wagon as two sub meshes of the same object, but the axle as a separate object altogether. When I make two or more meshes sub objects of the same object, I make some effort to make sure that those sub meshes are easily separately accessible. One way is to assign them to different vertex groups, but sometimes it is enough to just make sure that there are no connections between the sub-meshes, so they can be selected with CTRL-L.