Interesting. I always had two cannons once available in my field armies precisely because they were much longer range then anything else and would usually take out leaders and non-cannon artillery (like the horribly inaccurate onagers) easily. I didn't find them very useful against ground troops, though. Sure, the earliest cannons are very inaccurate but the later ones seemed great to me. Onagers? Man, I was afraid to use them as they killed as many of my men as the enemy.