My girlfriend informs me that the big difference is that in Birthright you get to scout previous areas and see if any enemies are back, and depending on the difficulty of the battle that comes, that determines how much it costs to scout that area, and then you get to fight another battle there. Good for an experience grind or to test out classes or build more relationships, but not essential. It doesn't sound like you really miss out on anything in Conquest, there's just that limiting option; from my understanding anyway