In mental ray for Maya, you can choose between No Deformation and Full Motion Blur. Motion Blur in mental ray for Maya blurs everything: shaders, textures, lights, shadows, reflections, refractions, and caustics.
The No Deformation mode results in instance motion; shape changes are not considered. The Full Mode additionally exports motion vectors for every vertex of the moving object. Use it to motion blur objects with deforming shape. This mode requires more translation and render time.
The No Deformation mode just exports instance motion performed on transform nodes, any shape changes (even linear movements) are not considered.
The Shutter Open, Shutter Close setting of the Maya camera determines the actual motion blur path length. You can modify other settings, such as Motion Blur By, to further control the final motion blur calculation. Find these settings in the Motion Blur (in the Render Settings window).
A typical application of motion blur is the Export Motion Segments option, which approximates any non-linear movement with a set of linear segments.