The reason they don't use 100% is because they're saving it for later.
Sounds silly, but stick with me on this one. Through a corner, especially in a race, you're making millions of calculations per second through all stages of the turn. You're watching the coarse (turn, apex, exit), the cars (what lines, speed, momentum), and as such your movements and decisions can fluxuate. You'll notice a lack of this when the competition starts dive-bombing into turns, or develops a case of target fixation. This is partially why some racers are only marginally faster/slower than their competition. They can't break free.
Knowing this, you've got to use everything, and I mean
everything to your advantage. In a turn, you aren't using
just your brakes. You're also using the tires, suspension, and body of the car. By saving the brakes, and modulating them as witnessed, drivers are able to judge where to be when, and what to do next. Those guys know where their competition is going to be before they do. It's like the Force. They save that 20% so there's something left when something else goes wrong, such as a spin or wreck at the exit of the turn. They make up for that 20% in other ways, yet never actually driving at 100%.
Driving at 100%, even braking, never has positive consequences. Locking up into a corner is never a good idea. Tires get worn, parts get broken, and if everyone were driving 100%, more people would be killed (not in the game, of coarse). What makes people like Sir Jackie Stewart, Michael Schumacher, and Ayrton Senna so great is that they could drive closer to 100%, for longer, and with the exception of Senna, live to tell about it.
Find the balance. Learn to induce. Maximize your advantage.
Have paradise,
and a chicken. (wife's advice)
GTP Racing School
No, not when your racing fast, you do always try and give yourself some mistake room under braking but not like that or that much.
You can't make up for the waste of 20% cornering grip usage on track, if you don't use it you have lost it. 20% under the limit is wayyy low for a fast room.
The only thinking that matters is the grip the tires can give you. Just because you can use 100% bake force, doesn't mean the tires have grip for 100% bake force, they may only have grip/stopping power for 40%.
Look up "Threshold_braking"
http://en.wikipedia.org/wiki/Threshold_braking
And Trail braking
http://www.drivingfast.net/techniques/track-corner.htm
If you use 100% braking power from speeds that only need 60% braking power (60% is the maximum amount stopping power the tire has grip for at that speed) , ABS will come in and remove the extra 40%. If ABS is "OFF" you will go over your maximum grip the tire gives you and crash.
You almost never use 100% braking power, it's actual slower. By only using the braking force you need, you maximize the amount of grip you have, giving you more grip for cornering.
Take a front tire coming into a corner. 100% grip will be used at all times, a fast lap.
Right now it is using 100% of it's grip to slow down. Say you're using 70% of your brake pedal (If you use 100% pedal then ABS will kick in remove your extra power and stop the tires from locking up because of exceeding the maximum % of grip ).
A second later you are going slower, so... less braking power is needed to keep the tires using 100% grip/stopping force. You can now start fading off the brakes (say 50% braking power, still using 100% of the grip that the tires can give).
Now your turning into the corner....
But wait, if you are already using 100% of you tires grip to stop, you don't have any left for cornering. If you turned the steering wheel now with ABS off, you will understeer (plow) into the corner and crash.
With ABS ON, it will kick in and remove braking power for you when you start turning the wheels. Leaving your tire grip too make the corner.
Some times it will be leaving you with
too much extra grip for steering or too little grip to make the corner (see ABS OFF result) .
This when you stop ABS from kicking in and start trail braking.
So you fade off the brakes more (30% say) and now your tires are only using 70% of their grip. Leaving 30% left for cornering! Now is when you would turn into the corner. Using up the "extra" 30% grip. Now your tire is using 100% of its grip for stopping and turning.
You need to judge how much grip you need to leave yourself for corners.
This is when a drive may "modulate" the brakes feeling how much grip they need to make the corner. A "Pro" will not modulate and smoothly fade off with the right amount of grip to make the corner.
Also, the more you turn the steering wheel, the more grip the front tires need. So, the more you need to fade off the brakes. You may need to start stopping sooner.
Some "PRO" drivers, drive using 99% of of the tires grip when cornering, others use 90%. The guy using 99% of his grip will be faster, but he is very close to crashing, and will wear out his tires quicker than the 90% guy.
I think any drive wasting 20% of the grip on track is not really a "PRO" driver.
Though, most of us here can't use 99% of the grip whenever we want to set a fast lap. That takes skill and track knowledge to push the limits.
EDIT: Based on a RWD car... FWD cars and 4WD/AWD cars can have added tricks.