In your example what's A and what's B? I'll be plain with you and admit I don't know what your getting at.
Look at your own example.
THAC0 requires subtraction which requires keeping the numbers straight.
BAB is addition, the elementary level commutative property applies.
In your "easy" example you have jumped to the (rather poor) presumption that the AC is always known. Yet in the BAB example the DC is unknown and must be checked against each time. You are still requiring the BAB player to be intentionally dumb while giving the THAC0 example player a the presumption of knowledge and correct procedure.
You are basing the arguement 100% on this stacked deck presumption.
Again, THAC0 is easy. I'm not arguing with you there. But BAB is is every bit as easy AND it is more intuitive to 90%+ of people.
To me THAC0 is easier because the math is a constant, i.e. for one opponent and one attacker it is always the same.
eg: x (The only variable in either equation) is the number rolled on the dice.
Using THAC0: if x => THAC0 - AC it hits
Using BAB: if x + BAB => AC
So if a player wants to figuer out what they need to roll with BAB, they can. But they need to rearrange the equation in their head.
( x+BAB => AC
x + BAB - BAB => AC - BAB
x=> AC - BAB)
Which certainly isn't 'intuitive' and is actually exactly what THAC0 is. (S'mon basically said this above, far more eloquently then I have.)
The trick with THAC0 is figuring it out before the roll is made.
You are talking about addition and subtraction with numbers in the 1 to 30 range.
It ain't rocket science.
If you don't know AC then BAB is easier because you can tell the DM your total and he knows if you hit or not. If you don't know AC with THAC0 then you have to tell the DM your THAC0 and the roll. And you must keep the numbers straight.
If you do know the AC, then it is 2nd grade math to get the target roll number under either system. Perhaps THAC0 is easier for C- level 2nd graders. I'm not so sure though.
But if the getting a target number out of BAB is difficult for someone, then I probably just can not relate to their perception of the system.
But to virtually every gamer I've ever played with, BAB is THAC0 presented in the way that humans actually think.