Here are my observations from testing in the 2014 release of AutoCAD Map 3D (I always test on the current release first) --
GRIPS = 0 deactivates all grips in a mpolygon.
Grips behavior is the same for mpolygon under GRIPS = 1 as GRIPS = 2. This is different from the behavior of polylines: when GRIPS = 1, the rectangular grips in polylines are disabled. When GRIPS = 2, the rectangular and square grips in a polyline are enabled.
There is a difference between polyline and mpolygon fundamentally - mpolygon objects do not contain rectangular grips. So I can see why they behave the same under GRIPS = 1 as GRIPS = 2.
The middle grip in a mpolygon is always for moving the object.
If you disagree with anything here, then that is fine. Just provide an explanation of what you are expecting and why, and I can see about reporting this to Development.