To answer your question a model does not need to be a single unified piece of geometry like a single perfectly joined polygon.
Here is a model that I made that is basically polygon boxes and other primitive objects all stuck into each other. This is common for these type of models (see Kurt B's assault mech tut).
That being said the topology should be clean (no floating verts, non-manifold geometry, co-planar faces, faces with zero surface area, etc. These are the things that cause Maya's tools to malfunction and others done the pipeline to want to put a hit contract out on you!
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675