AI in some of the previous C&C games used some game-breaking cheats to gain an upper hand. While understandable in some cases, they went a little overboard.
AI should not obviously cheat. It's one thing to give them a couple advantages to make them harder to play against, but it's another thing entirely to give them game-breaking cheats. You can give them a slight increase in income or lower power requirements or even quicker build times... but direct actions taken on the field should be left to the AI's intelligence alone, without the assist of some cheat.
AI should have to recon the area before noticing one of your attack forces. You, as the player, should be able to outsmart/outmaneuver your AI opponent... but you wont be able to do that if they know every move you make. The AI in RA3 was notorious for knowing exactly where every one of my attack forces assembled... even if they were way out of their field of vision. I'd group a naval fleet in the corner of the map, and the enemy would always send a helicopter straight there... it was quite annoying.
If you have a unit that has an "invisibility" ability (like cloaking, or burrowing, etc) the AI should not know of their presence without the aid of some sort of sensor building/unit. In C&C3, I would have a stealth tank poised to attack an enemy, but the enemy AI would know where I was and follow my unit around waiting for me to show myself before they opened fire.
-Makes the AI more fun to play against.
-Allows you to practice game strategies on the AI if you aren't up to playing in an online match.
-Could potentially make the AI easier to defeat... depending on how much work Victory Games puts into coding it.