The use of brakes in any corner is fine as long as you use good judgement. When brakes are applied regardless of lean angle, road condition, weather, local taxes, or your need to pee, the bike weight shifts to the front wheel. the harder you apply either brake the more the weight shifts. If you use the front tire only you can overwhelm the limits of traction leaned over and down you go, if you overdo the back brake the rear tire unloads and tire lockup can occur losing traction and down you go. The key is not over doing it on either tire and that takes good judgement and some experience. The real reason braking is discouraged is because the steering gets really heavy and handlebar control becomes like wrestling a bull by the horns, it tends to take you where you don't want to go. The ratio I live by is 85/15. The front is good for 85% of braking and the rear 15%. That means if you're going to use the brakes in a corner to trim speed, start with the rear and add front if necessary. Of course the best answer is: don't go into a curve so hot you challenge the laws of physics and your own riding capabilities.