You bring up several good points. At Giro we're also pushing for an international standard that addresses rotational energy, and as you mentioned every technology in this space comes with a trade off. In the case of MIPS Spherical, however, you are mistaken about the "squishy rubber whozits" between the shells. In this system there are simply two shells (like a ball and socket). Both surfaces are polished/slippery so that they can rotate independently without much force. The two pieces are held in place by the rubber whozits that you mentioned (we call them elastomeric attachments), but they're not in between the shells, they're holding the pieces together and tuned so that you don't have excess movement while riding.

There are a number of plus sides -- the system doesn't add weight, doesn't impact comfort, doesn't impede airflow. The one noteworthy downside is that it adds complexity (in other words cost) to the helmet.

I am really glad that you mention the potential of a larger/heavier helmet increasing the chance of concussion. There is a common misconception that a moto helmet is better for MTB. Moto helmets address high speed/catastrophic crashes, but they don't perform as well in low speed impacts (especially when rotational energy is introduced) because they use a more dense foam and as you mentioned, they're bigger and heavier.

I'm proud to work for Giro and see the energy this company puts into research and development. The quest for improvement is never over, but the Tyrant here represents a milestone in head protection if you ask me.