Testing by various paladins reveals that the Pursuit of Justice talent (8% to movement speed) stacks with mounted movement enhancements, but does not stack with ground movement enhancements. So PoJ is a weaker talent than a lot of the other talents, given that you can just put a speed enchant on your boots and get most of the effect.
However, there may be some areas where the increase to mount speed is useful. For example, in Arathi Basin, I notice that the Alliance almost always caps the Stables before the Horde caps the Farm. Yet the Horde almost always gets to, and often caps, the Blacksmith before the Alliance arrives. I'm not sure why this is--perhaps the route from Horde base to flag is slightly shorter. Regardless, a paladin with PoJ might be able to arrive at the flag fast enough to stop the Horde from capping. As well, there might be some advantage in Alterac Valley, as a paladin at the base will be able to get to the front lines faster.
Of course, the stacking with mounted movement might be unintended, and fixed by Blizzard in an upcoming patch.