My view on this is that it will be best to separate the head and body meshes because it will make swapping heads in and out easier for those who wish to customize or animate their avatars. This will be especially important for "furry" or other non-human avatars, since the primary difference is the head on such avatars. Likewise, not having hair integral to the head mesh is also desirable by most residents since they want to be able to customize the hairstyle on one's head.
The only problem is that the joint between the head and neck may be visible if not fitted well, but I doubt most people would notice or care.