The reason it's not preferable is because with the manual in nuetral, and the rear wheels spinning, that mainshaft of the trans will spin in place without lubrication.
The way it's normal lubricated is by the countergear splash lubricating everything internally as it's driven by the input shaft connected to the engine. Since the engine is off, and the input shaft not turning, no splashing of lube.
How much or little damage gets done at that point is anyone's guess. I do know that when starved of lube, specific older model manual mustangs did gall up the mainshaft creating lots of whine. I would personally disconnect the driveshaft here. They have driveshaft disconnect kits specifically for this.