Should tf frames exist at all times?

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I am interested to know from the authors of TF if the nodes/joints in a tf tree are expected to exist during all the time the robot is up and running?

As an example: using tf to spawn new joints (for example object grasped will have its tf spawned, as a child node of the gripper) and then let it disappear once the grasped object is put down. Or imagine two robots that could dock to each other (like a locomotive and wagons) and then a link would be spawned between both robots.

would that infringe the rules of using tf? What would speak against it? And what would be an alternative solution in that case?