The question is: Where do we draw the line in sport inovation? Also, if one sportsman (a runner) is allowed to use better and better running shoes why not another athlete (a swimmer) with a better suit?
Any line is essentially arbitrary - there's no real "should" about it. Factors that are involved in making a decision include safety, social values, corporate sponsorship, the purpose of the accessory, and the spirit of the sport.
That last one is tricky to pin down, but I think it comes down to what people think that sport is supposed to test.
If it were only about the spirit of the sport, then athletes and swimmers should perhaps perform naked.
Corporate sponsorship is easy to dismiss as money-grubbing and degrading, but it does have value in promoting technology innovations (look at the motor vehicle technology derived from racing competition).
The running shoes analogy is a good one to investigate the purpose of the accessory.
What's the difference between a legally technology-enhanced shoe and a roller skate or a swimming fin? Why are the latter two illegal?
The answer lies in their purpose. The purpose of a running shoe is to protect the foot and ankle of the athlete and provide grip. The purpose of a roller skate is fundamentally different, as is the purpose of a swim fin. According to the "purpose" argument, any technology enhancements must not significantly alter the purpose of the accessory.
That's why there is controversy over these swimsuits. The purpose of a swimsuit is to cover the swimmer's nakedness. An advantage such as streamlining could be considered incidental to this purpose, so it makes sense to make them as streamlined as possible. But what about buoyancy? If a swimsuit is made to buoy up a swimmer, does that significantly alter its purpose?
It's questions like these that lead to rules defining what is and isn't allowed
