I have another thought about the strokes. Knowing a variety of strokes is very helpful in an open water swim. There will be times that a breast stroke is more appropriate than the freestyle in the swim with all those kicking people around you.

Another reason for swimming different strokes is to use different muscles and avoid getting injured from doing the same stroke all the time during training. This is particularly important if your stroke is not completely correct--you could be doing one little thing that if done over and over could lead to injury.

Finally, it's just downright boring to do the same stroke all the time.