You're absolutely right about the importance of rest while training. It's possibly the most important factor in making gains in fitness; providing the appropriate energy systems with just the right amount of overload, and then letting them adequately recover and get stronger. You don't get faster by training, but by resting after training.
With three sports to train for, it can be very difficult for triathletes to structure enough rest into their program, and many fall into the trap of exercising every day, even on rest days. However, the purpose of rest isn't to simply stay off of your legs, but to allow the
I believe all athletes should incorporate a complete day of rest once every 7-10 days. The positive impact of one full day of rest outweighs the potential benefit you would see from adding one or two more workouts to that seven-to-ten day period. You'll come back feeling stronger, having better workouts, and enjoying training much more.