I have kind of run into this too. When I sent our engineering dept simple extrusions to open with Inventor the other day they had to play with the import options for a while. This is what I was told.

"With the default setting the geometry imports as surface data. If I turn on the Auto Stich and Promote options prior to import, they import as individual solid bodies."

I don't know anything about Inventor (or any other solid modelers than MoI) but I would just assume a solid in one program would be a solid in another. Maybe it's more of a problem with Inventor than MoI.