Isn't Challenging a full AI without bonuses?
If it isn't, then I think we would be better off having a difficulty level with the best AI, but no bonuses instead of splitting the options.
That said, the situation isn't as simple as one might think. Some things are just incredibly difficult for the AI and some small help - not a bonus giving numeric advantages - may be necessary in order for the AI to function. Typically these include Fog of War cheats. Besides, humans cheat way more than AI by autoreloading, using Ctrl-N to create a starting position they like, picking their opponents and so on..