This is the case for riders of all experience; beginning riders often struggle with the technique, and will often simply not combine any braking with lean angle. At the other end of the spectrum, though, more experienced riders realize that it is an important skill and a necessity for quick lap times around the racetrack. For an action that comprises just a few seconds of the total lap time - if that - it demands an inordinate amount of attention. Why is this?
The main components of a quick lap time are acceleration, braking and cornering. To go fast you must accelerate at the maximum rate possible, brake as hard as you can, and carry as much cornering speed as possible. These actions do require a certain amount of skill, but eventually there are limits based on the tires and the motorcycle; even moderately experienced riders can get quite close to those limits.
Anyone can hold the throttle wide open on a straight. Yes, hard braking is difficult, but the warning signs that you are approaching the maximum deceleration are relatively clear, and the limits are defined by traction and the motorcycle tipping over. Finally, and somewhat surprisingly, the difference between expert riders and not-so-expert riders in terms of absolute corner speed is not that great - I have followed more than a few first-time riders at the track and held my breath as they carried more corner speed than I was willing to.
Once the limits of cornering, braking and acceleration are reached, this leaves the transitions between those components to work on. Between braking and cornering is trail braking, and for lack of a better term I call the transition between cornering and acceleration "trail acceleration." There are other transitions, although they occupy only a fraction of a percent of the entire lap: going from acceleration to braking at the end of a straight, or going from full lean in one direction to full lean in the other.
Accelerating out of a corner while leaned over is, of course, a necessity. But as tires and chassis get better, and traction control becomes more prevalent, finding an advantage in this area is becoming increasingly difficult. On a motorcycle with moderate power output, you can apply full throttle and maximum acceleration at a surprisingly high lean angle. On more powerful bikes, the trend is to stand the bike up at the exit of the corner and get to maximum acceleration as quickly as possible. The time spent in the actual transition between cornering and acceleration, and the potential gains from mastering this transition, are reduced.
What's left? More emphasis on the entry to the turn and trail braking, and this is the skill that often distinguishes good riders from really good riders. It's interesting to note that in series with spec components and motorcycles with only moderate power levels, such as Moto2 or any of the middleweight supersport classes, riders often talk about corner entry and how that aspect of performance is the most important. Even in MotoGP, with spec tires and elaborate electronics, this is often the topic of discussion.
Using data we can look at these transitions, and put numbers on them. How much braking is the rider combining with cornering? At what lean angle does he reach full throttle? How quickly does he go from throttle to brakes at the end of a straight? At some point, of course, these skills are mastered and the next level must be investigated - the transitions to and from those transitions, and how quickly they are occurring.