Motion blurring is an important addition to most animations, because it helps fool the brain into thinking the sequential images it sees are moving. Even the simplest animated scenes look more professional with a bit of basic motion blur during rendering.
This post-production shader was created to solve a specific problem - the length of time it takes trueSpace to render multiple frames for motion blurring. It is a compromise; while the tS motion blur can look between whole frames and thus get a more subtle result, motrails saves an incredible amount of render time, because it looks at existing frames without having to render more. A 200 frame animation that took 14.5 hours to render with tS motion blur took just 1.5 hours with motrails - 13 render hours lopped off!
There is another way of using motrails, you can render your entire animation without the motrails shader, then use that render as a backround in an empty tS scene. Render this new pseudo-scene with motrails applied. You could even use video footage as the background, and apply motrails to that!