An ambient light in reality is just a brightness wash applied over the render, and normally destroy a good looking render if used incorrectly. Maya´s ambient light does differ from the pack since it does compute the position of the light, however not as much as you would perhaps like.
That´s why the bumpmap isn´t computed within the ambient light.
Go through Mike´s three point tutorial
to get a good sense of lighting a 3d scene. That way you´ll preserve the bump map, and achieve a fairly better render.