Revision history [back]

It's not a real answer but a bit too long to be a comment sorry.
I don't know for the two first ones, I asked myself the same question about the first one and I would be interested to know what I missed.
Regarding the last one about transmissions, I think you're right about the fact that it is used to specify the kind of controller used.
I my opinion the main point of this tag is giving a true physical meaning to how a motor can actuate a joint. As far as I understand, this is the purpose of the actuator tag. This allows you to specify the reduction ratio that is indeed applied on the output of the motor. This way if at some point you can define exactly the characteristics of your motor (that you cannot really do yet), this tag will allow you to have reliable torque estimation applied on your solid (links). This way you should be able to have finer simulation regarding the behaviour of your joints according to the mechanics and electronics interations at the lower level.

On many robots, the same motor model is used to actuate different joints and the only difference is the reductor putted between the motor and the actual moving part. This tag should allow you to make the difference between a given motor model and its real action over a given joint.