Right, motor control doesn't come from the same area of the brain that memory does. Memory is mostly located in the cerebellum, and while it's distributed across it, even minor damage, such as a concussion, to the cerebellum can cause permanent memory loss (I know this from personal experience, I suffered a concussion in 6th grade and still don't remember the day it happened or much of the preceeding 2 months).
When you're talking about complete brain death though you're talking about 100% loss of memory and function. If the brain dies, and the cells within it die, it doesn't matter if you generate new cells using stem cell treatment or whatever else. Once those cells are dead, all the connections they had with other cells that form the neural networks which store memory and personality are gone, completely wiped out, irrecoverable. The only reason brain-dead patients are still "alive" is because the functions of the brain that regulate breathing and heartbeat are replaced with machine stimuli. The body is only alive in the crudest of senses.
The same goes for extremely traumatic head injury where the patient is in a coma and 99% braindead but some simple functions remain; the more critical to life a bodily function is, the deeper in the brain it's located and the less liable it is to be damaged by injury. The parts of the brain that store memory and personality are just as dead, useless, and unrecoverable. All that's left are some of the systems that regulate very basic functions.
