After you play MTW enough you come to realize that you don't need more than a full stack to win a battle, even against the Mongols (if you catch the initial assault). Holding ground is different. This is especially true if you're defending in a hilly or mountainous region. Ya, epic battles are cool but they're also...epic. This means a long, rare struggle. Increasing the number of them will decrease their importance (It's like having 70 virgins; trust me, you'd get bored of that quick).

You don't want to have to constantly defend yourself against extinction. Instead you want challenges and surprises that make victory more rewarding. Besides, the goal should be to avoid those struggles so you don't reduce your army or treasury to ashes.