First there are more than just one reason why not to make alot of reps. If you train like BBilder with 8/12 you provoke unnecessary grouth. Your strenght wont increase and your endurance will go down... Plus you smokes your muscles. This means you wont be able to do alot of work in one sesion. If you make 12 repetitions with small weight you fill this burning in your muscles aren't you? This is bad for strenght. This means that the levels of lactic acid are going up and you get tired. We don't want that!

Second going to failier provokes your nerve system to reduce your power. This is precotion not to get hurt. Thats why is much better to stop 1-2 repetitions earlier!