There is an issue issue in Maya 2012 hf2 when the Map Visualizer is enabled and final gather map is set to off.
As a workaround we can force the legacy finalgather map containers to be used.
1. In the command line run "select miDefaultOptions;"
2. The attribute editor will load some mental ray options. Scroll down and expand the "String Options" category and click "Add New Item"
3. Scroll down to the bottom of the list to see the new empty options. Enter the following.
Name: finalgather legacy
Afterwards you should be able to set your scene as you normally would and the maps should work with final gather rebuilt off when the map visualizer is enabled..
Thanks to Ash for his contribution.