Why? Each drill is designed to teach some skill. The skill that is being taught works against even best play defense. A less than best play defense and you don't learn the skill...
Ability to Adjust the Computer for Drills

Actually now that I think about it, I see the appeal especially in the "odds" section where you may simply not yet have the skill to win. I don't agree that it's a good idea, but I do see why you might want it. Even a crippled full strength computer puts up a ferocious battle that could be demoralizing.

I catch what you trying to get at. My view is when you learn boxing you start off with strength and conditioning, Learn to punch. Learn to hit the bags and focus mitts. Then you Spar. Then your on the Under Card and maybe one day your the Main Event on HBO. It's a progression of Skill.
When you went in the Service they just didn't Hand you a Uniform and send you out in the the Fleet. or Over Seas. A game tough game like chess, it has taken me years to just reach advanced beginner.
It's a progression of skills.
I would suggest to include in the Drill's an ability to adjust the level of the Computer for drills such as when playing a game against the computer.