With all respect, I think that the two are very different. I think the damage to a beardie, caused by shaking it by the tail, would be much different than shaken baby syndrome...
After thinking about it for another minute, or so, I was going to retract that, as it is probably possible to cause similar damage if the shaking were violent enough....but then I paused again for more thought. I think if you were to shake it violently enough to cause the neurological damage seen in shaken baby, the tail would probably break off in your hand...sending the beardie flying across the room. (little light bulb goes on) Maybe
THAT'S why the vet said to hold it by the tail - to serve as a failsafe against the possibility of causing shaken baby syndrome!! This makes much more sense now