If your goal is to get a workout (burn calories, raise your heart rate, move your muscles, etc.), then why would you want to learn to swim more efficiently? Wouldn't it just take more time to achieve the same amount of exertion?
Of course I understand that if you want to win a race you need to be efficient, but it seems to run counter to the goal of exercising. No?
http://en.wikipedia.org/wiki/Anaerobic Some beginner swimmers (including myself before TI) swim anaerobically (lactic acid produced, pain in muscles). Only with better technique will you be able to swim aerobically (laps).
"Olympic swimmers are able, at best, to convert only 9 percent of their energy into forward movement while swimming the front crawl or freestyle, the fastest and most efficient of the four competitive strokes. (On average, the butterfly, backstroke and breaststroke follow in speed and efficiency.) Most of the rest of us use as much as 98 percent of our energy overcoming water resistance. " (2)
"But the real killer for competitive swimmers is a third type of resistance that arises at the interface between air and water—wave drag. Moving along the surface of the water inevitably creates waves. Physically speaking, swimmers force a mass of water in front of them to rise up against gravity. This not only robs swimmers of energy, but it has a disproportionately greater effect the faster they go.
The problem is that wave drag increases as the cube of any increase in swimming speed. And it gets worse if a swimmer makes jerky or uneven movements, either bouncing in the water or moving from side to side, because this wastes still more energy making waves." (1)
(1) http://environment.newscientist.com/article/mg15921455.600-s...
But it feels fantastic to move efficiently. I don't know if that gives a better physical benefit (though I bet it's a factor) but it provides motivation. And that helps with the most crucial factor of all for effective exercise: to do it.
A lot of swimmers do interval training too - eg 20 by 4 lengths every X minutes (with X including some rest time) - as you get better you drop the minutes to give yourself less rest and can up the number of intervals too.
I guess its all down to enjoyment to swim more efficiently, as you counter the energy savings by going faster and longer ... Its like the pleasure you get from programming in a language you are expert in vs the one your just learning (which is rather frustrating until you get good at it and don't need the manual every two minutes!).
Swimming efficiently also is more of a full body workout I think than just paddling your arms.
Also swimming poorly can be frustrating and would make people not want to work out as long or maybe give up swimming alltogether.