My expertise is not in X-Ray tubes but after doing some fairly in-depth reading recently (for academic study I am undertaking); the use of a liquid metal lubricant is preferable for very good heat and electrical conduction in heavy-workload (Fluoroscopy) X-ray tubes, in particular, that need a high loadability.
Wear is much reduced because the rotating anode is never in contact with bearing surfaces, only the metal lubricant (a metal, gallium alloy, that is liquid at room temperature) circulated within a grooved sleeve bearing that floats on the bearing-liquid. Wear only occurs during braking and acceleration, apparently. That is why these anodes are kept spinning continuously (operating mode in fluoroscopy systems used in cath labs, etc).
The liquid bearing has low vapour pressure that helps to maintain the vacuum conditions in the tube. Potentially longer life, due to reduced bearing wear, higher loadability, in terms of thermal workload, therefore improved reliability are the benefits, in theory, I think.
