Hi Hoschi,
make sure you don't have rgbd_odometry running if you are using robot odometry. Using fake laser scan from kinect won't give better maps. It could be used for local costmap of move_base though (faster than processing the whole kinect cloud), and rtabmap can create 2d maps faster with a fake laser scan. If you have a real laser rangefinder (>=180 degress of field of view), then there is an advantage to use it with rtabmap (more accurate loop closures and odometry correction if RGBD/NeighborLinkRefining=true and Reg/Strategy=1).
A black odometry window in rabmapviz doesn't mean that rgbd_odometry is not used, it just means that rtabmapviz's "subscribe_depth" parameter is false or it is not receiving camera topics. Use "rqt_graph" to see all nodes running. If you are using
rtabmap.launch, "visual_odometry" argument should be set to false to not use rgbd_odometry.
cheers,
Mathieu