You might think that you're saving polygons but the amount of vertices(which is what really matter in an engine) is the same, or even more sometimes. Smoothing/UV splits add to vert counts as well. But leaving meshes open can certainly make modeling easier(and save unwrapping time/space) and of course faces that will…
I've found that, when rigging and animating humans, sometimes it's easier to connect some parts even if they are hidden. Here's some pictures. Easier to see than explain : Originally, the chainmail arms were a separate mesh just inserted inside the torso. This made rigging more difficult though, as I had to get inside,…