Other stuff like missles and airplanes can also leave and reenter the map.
It would be a bit strange if an airplane outside the map takes fire by flak but you don't notice because you can not see the explosions...
I think its actually good that gfx aren't culled. It provides a sense of a wider world. Plus culling gfx would look really weird (imagine Argh's nuke detonating at the edge of a map)
