I have a few thoughts on the AI's use of cannon.
1. The enemy tends to build to many, typically 6-8 units in a 30+ stack with many smaller stacks holding as many. This leans me towards arming myself with at least 6 and both detracts from the more interesting gameplay of infantry numbers.
2. I see the AI forms one single battery...as mentioned this will normally be 6-8 units in one line behind the center of its formation (a straight line). Its quite daunting to see the barrage, but in all honesty its never to hard to get in behind that line due to the cannon normally remaining static as the infantry moves forward. The result is one single unit of cavalry can destroy all the AI's cannon in seconds. (normally my fist point of call regardless of cost!).
If the cannon was formed in either single units or doubles and scattered around there formation, apart from being more realistic, it would be much harder to target the said cannon, for you would have to find them first and get in behind their units in many locations.
Just my thoughts...