In that specific context, no there is no reason to use post. But in general, are you aware of how the order of deformation affects things?
In audio, if you connect 2 effects to a guitar, 'distortion' and 'delay echo'. Depending on which one you put first, you'll either here a distorted guitar echoing, and each echo will be identical, or you'll hear a clean guitar echoing and then being distorted on top. So each echo could be distorted differently. (insert useful illustration here.)
Post-deformation could be useful for doing some global-space deformations. Off the top of my head, maybe you want a shape that stretches your character up in Y to help mix with some kind of teleportation effect. If it was in pose-space, and their head was rotated, the effect would move up in the direction of their head.
Excuse the crappy drawing. I blame Gimp....
But now keep in mind that you can also animate blendshape sources, AND you can put blendshapes on things like curves and lattices, (not just your final character) and now you can begin to invent useful, practical rigging reasons why you might want world-space deformations on something.
For example, a squid-like character has tentacles that are being driven by spline IK or ribbon IK. You can put global non-linear deformations on those ribbons. And if you want to mix multiple deformations, sometimes it is useful to mix them in using blendshapes. Because then you can paint their influence weights, or control where in the history stack the deformation occurs.