Swimming strokes besides freestyle when training for a race has several advantages. Let's look at some reasons to work the other strokes and a sample workout for your next race.
Repetitive Motion Injuries
The major problem with training just freestyle when you are doing 3,000 to 5,000 yards per workout is shoulder problems that can develop. Too much of one stroke at that distance consistently can be problematic, especially if there are any flaws in your stroke.
Backstroke Helps With Hip Rotation
Learning proper backstroke and mixing it in to your workouts can help your freestyle hip rotation. It is even more essential in backstroke to rotate your hips with each stroke than in freestyle, and will help your brain make the biomechanical connection. If you're only going to learn one other stroke besides freestyle, backstroke should be it.
Boredom!
Swimming thousands of yards and hundreds of laps of nothing but freestyle is way too monotonous and can play with your mind! I recommend mixing it up. Backstroke or breaststroke can be relaxing. Butterfly can give you a nice ab workout, and even just one or two lengths can burn many calories!
Individual medley (IM: butterfly, backstroke, breaststroke, freestyle) can really give you a good aerobic workout, take your mind off of counting laps or yards for a bit, and relieve the boredom of staring at the line at the bottom of the pool.
It's important to take a "holistic" approach to your swim training. If you are just training to do one sprint-distance race and nothing more, ever, then training strictly freestyle should be fine. Otherwise, I would make it a mission to at least learn backstroke, and practice it with one workout per week.
Here's a sample workout with strokes mixed in. You can always substitute backstroke for any stroke you don't know.
Warm-Up: 300, every third length non-free
Drill: 5 x 100, 10 seconds rest in between
- Odd 100s: four strokes back followed by four strokes free
- Even 100s: Freestyle, think about hip rotation
Main: 7 x 200
- No. 1-3: Freestyle, descend time (fastest on No. 3) on your base or cruise interval
- No. 4: IM (two lengths each, Fly, Back, Breast, Free). 30-seconds rest
- No. 5-7: Freestyle, descend and lower each interval
Cool-Down: 200, breathe on weak side for odd lengths
Learning the other three strokes—butterfly, backstroke, breaststroke—is, of course, a personal decision. You don't have to learn anything but free for triathlon. But as I've stated above, there are some nice advantages to doing so.
If you decide to take on one or more of the other strokes, take your time. Think of it as a form of cross training. There is no hurry, and keep in mind the results will be improved freestyle technique, more speed in the water, more fun training and fewer injuries.