@ Believable--Sorry!  I didn't know which program you were exporting from.  I can only talk about problems I'm familiar with.  In Wings3d, and other modelers I have, it's impossible to export the ground plane, and in Poser it IS possible.  I never thought about welding or not welding vertices because I always deal with single actors, not the whole mesh.

I cannot save the world. Only my little piece of it. If we all act together, we can save the world.--Nelson Mandela
An  inconsistent hobgoblin is the fool of little minds
Taking "Just do it" to a whole new level!