->Toomas-Jaco - you right
1-When bake Ambient Occlusion with mental Ray Occlusion shader for light(NOT Global Illum. like lightracer do, ITS DIFFERENT Things) in objects properties of your low-poly UNCHECK in Render control all radio buttons exclude Renderable(if ucheck- bake to texture will fall everytime)
Your low-poly will renderable but not visible in render frame window(is it bug of MR working with BakeToTexture?)
2- Ambient occlusion is procedurical shadow, its show places where light is attentuate(fall off?? Sorry my english Im from russia))) and its work something like dyrt, its not lighten deep places even light point directly cast light to it, GI instead can lighten deep places of your object.
AO generated by every part of your model? in max you can setup spread angle, the more rays casted from every point of your model hit surface- the more darkly this part of surface. Flat surface with spread <0.9 will bright, more 0.9 it will self shadowing(dont use more than 0.8
Thats why Ambient Ocllussion more preferable then GI type of light.