What you define as 'resource cheating' is like 'boost balancing' in racing games. The 'boost' is used to try and keep the pack in relatively close proximity to each other to create the illusion of a tight race. If the player isn't in the lead it may keep the CPU player in sight, and if the player is in the lead it keeps the pressure on by putting opponents directly on your tail, even if the CPU itself is a mediocre driver.

In an RTS you're looking at a collection of values which can be arranged optimally. You know exactly how long it takes to obtain a resource, how much building items costs, map traversal times, etc.

If you're the one who has created the tables and has the info readily available to you it's just a matter of number crunching the data to find the best ratios for output.

But how much fun is it to play against an enemy who always does exactly the same thing based on the context of the playing field?

So you add chaos. You give the AI an efficiency on a sliding scale. You have it harass the player. If the player appears to be doing too well, you have the AI cheat a bit and give it some extra resources or reinforcements to keep the player on their toes.

You want the player to feel the satisfaction of facing a challenging opponent, rather than spanking a weak AI or being slaughtered by Hal9000. The perfection of a CPU opponent lies in its imperfection. :)