Manual transmissions are more common in the UK then they are in the US, so the yellow warning light also helps people to know when to shift back into gear. I found myself frequently eyeing the other directions light when on my motorcycle for this exact reason.

As for hybrids, not many completely shut down the gas engine. Toyotas are the ones that usually do, while Hondas tend not to. Most of the savings comes in not needing to use gas to get the car moving initially, as the electric motors are far more efficient in this area (due to torque). Since most hybrids also capture energy when braking, stop and go traffic can be handled by just the electric motor.