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.?