You don’t have to become an expert in both roles. But, experiencing the dance from the opposite perspective teaches you how lots of little details—from hand position and distance to prep timing and settling weight transfers—can dramatically affect how pleasant the dance is for your partner as well as how many options the partner has in a situation.
There is no cure for arm leads or coaster steps on the anchor as powerful as having the erstwhile leader feel his arm being jerked around or letting the former follower experience her follower charging at her on 6.
In fact, learning the opposite role is such a powerful teaching tool that I will say: a dancer should begin learning the opposite role as soon as he or she feels comfortable with the basics of the dance in their default role.
Learning something from musicians
This opinion comes from my experience as an oboist.
The oboe is a double-reed instrument, and the reeds can be made harder or softer by altering the thickness of the reed halves.
Although most oboists start using store-bought reeds, after a few years their playing is limited because purchased reeds are never a perfect match for the hardness required to match the player’s mouth embouchure.
At this point, oboists start learning how to make their own reeds, which they can tailor to suit their own playing preferences.
However, making reeds is extremely difficult, and there is always a period of a year or two in which the oboist’s reed-making skill is not able to keep pacing with his or her playing skill.
Eventually the oboist improves enough at making reeds to consistently produce better reeds than could be purchased, but that transition time is awkward because the oboist is limited by both the quality of the store-bought reeds and his or her own reed-making ability.
This transition time is one of the most significant limitations to the development of the oboist’s playing skill—after the oboist gets through this period, his or her playing ability usually improves dramatically because the reed no longer holds back the playing skill set.
The same thing happens, to a smaller degree, with learning opposite roles and essentially degendering west coast swing.
Although it is possible to keep learning one role without learning the other, it’s harder to pick up the nuances of the dance unless you know what your partner is experiencing as you lead or follow. The sooner you start learning the opposite role, the sooner that you can use that knowledge to accelerate your growth in your default role. If you are comfortable with your basics, there is little risk that learning the opposite role will confuse you.
Hence the recommendation to start degendering west coast swing by learning the opposite role as soon as you are comfortable in your default role.