Triathlon training is not single sport training. It's therefore virtually impossible to be the absolute best you could be in just one of the sports if you are properly training for triathlon. Athletes can certainly get very, very good in all three at the same time. But not the best. So, it's sometimes advisable to tackle a single sport for a training period or two with more enthusiasm.
For most age group, amateur athletes, time is the limiting factor in training. Between families and jobs, it's virtually impossible to match the schedules of those who can focus primarily or exclusively on athletic performance. So you must approach this with a N=1 type of attitude.
After a season or two of triathlon, it may be time for you to consider devoting a training block or two to one specific element. Typically, in newer athletes, this would be an athlete's weakest sport. The thought is that for athletes in early development, they will reap long term benefits from elevating their weak link sooner than later. If you can finesse the technique of swimming, or pump up your cycling legs early in your career, then, long term, you can adopt a more balanced approach to the three main sports.
Athletes who are further along in their career might switch to single sport focus for the same or different reasons. They could need a mental break from strict triathlon training. Or they could have a desire to meet some specific milestone/goal that has remained unobtainable to them.
Using a single sport mode could also be seasonally related. Not everyone has the ability or mindset to use indoor trainers most of the time. An easy example here would be for northern latitude athletes to focus on running in the winter.
All this being said, as fundamental triathletes that means there is an element of cross- and maintenance- type training that can and should go on in any single sport mode. Strength and conditioning benefit all sports. Riding, running, and swimming all help develop and maintain aerobic volume and conditioning.
Single sport training doesn't mean you have to stop your other sports. It just means you're focusing on that sport for a specific period of time with a specific goal to you. If you time it well with your overall athletic life calendar, this focus can be a great competitive benefit as well as a good way to retain interest in triathlon training over the long term.
Some quick examples would be:
Road race training in the fall/winter
Bike racing/training in the spring or fall
Swim training in the spring or summer
To reiterate: You can do all these while still "triathlon training." You just are very unlikely to do them as well as you could if you truly focus on that element for a while.
Marty Gaal, CSCS, is a USA Triathlon coach who lives in the Triangle area of North Carolina. Marty has been coaching endurance athletes since 2002. You can read more about OSB coaching services at www.osbmultisport.com.