In general, V-Ray is so well multi-threaded, it makes little sense to attempt to run multiple tasks on the same machine.
Concurrent Tasks in 3ds Max are a good idea when the computing operation is mostly single-threaded. A good example is simulating Particle Flow and saving to disk (e.g. using Krakatoa Partitioning). Particle Flow is inherently single-threaded, so multiple instances of 3ds Max running side-by-side can make a better use of available cores.
Multiple Slaves are useful for running heterogeneous job types. For example a CPU heavy task like rendering or simulation together with an I/O heavy task like compositing in Nuke , AE or Fusion. In most cases, the render job will have short bursts of I/O activity loading assets, and then long stretches of 100% CPU load. At the same time, compositing tends to be constant I/O activity with less CPU use (and the larger the resolution, the worse the CPU load!). So running two jobs of different types on the same hardware would saturate both types of resources (CPU and I/O) better than running one job only. Of course, each job will perform a bit slower, but if you would compare running two render nodes where the one node renders and the other composites, vs. two render nodes where each runs a render and a comp, chances are the total performance of the latter setup will be better than the former…