I'll give it a shot too, here. I think you have to separate the issue into two categories: racing and street (daily commute or whatever) driving. First let's talk about regular old street driving.
Scarpa was right on about control, that's exactly what it does for you. The bigger rotors help prevent fade my dispersing the heat across a bigger area, and the larger calipers cover a wider clamping area. So you get better clamping with less force, and your pads don't fade as quickly.
This is all about control, and if you only have to push the pedal down 1/2 as far with bigger brakes, then that gives you much more control over your own destiny.
The tire lock-up problem is just a traction issue. The idea behind bigger brakes, at least in the context of daily driving, is more control. Sometimes just having a better pedal feel is worth it.
Now to talk about racing a bit... the BABK was designed by SMC for racing purposes. It's overkill for the street, plain and simple. Most people like the added control of the AD22 brakes on the street, but I never needed any more than that with my NX, at least on the street.
But on the track, you need all the braking power you can possibly get. If you drive hard and brake late, your AD22s will fade, and you won't be able to lock them up. BTW I'm talking about road racing, not autox. The AD22s are actually probably fine for AutoX.
The BABK has 4 piston calipers and almost 12" slotted rotors. There is enough there to keep most SE-Rs happy on any track. Last time I went to the track I had Hawk Blue pads for my BABK, and I found that I could not even heat the pads up hot enough to get them into their comfort zone. As a result they ate my rotors all to hell. And believe me, I was late braking as hard as I could. I even got fluid fade, but I could still lock up the tires (sticky race tires) at 95 MPH.
So that means that the brakes are good enough that I don't even need full-out race pads, even driving as fast as I can.
With race tires especially, the stock brakes won't cut it. The AD22s are OK to a certain point, but generally the tires can out-grip the AD22s. Rob Cadle uses Hawk Blues with his AD22s, and he says that's just about right, he likes that combo. But he also has some fade issues, and he's right at the top of his brakes' capacity. The BABK affords more control because it dissipates more heat and has a lot more clamping force over a wider area.
In my case I bought the BABK because I knew I was going turbo and would actually need it. I expect to tax my brakes at the track quite a bit more when the turbo is done. As it is right now I can't even use their full capacity.
And to answer your question about locking the brakes at 70 MPH... how many times in a row can you do that before the brakes heat up past usability? A lot of that has to do with the pads you're using, but bigger brakes will definitely allow you to do that more times in a row... if that's something you like to do a lot.
Hope some of that made sense.
