Today I had the evening at a longer session (two and a half hours) on the indoor bike the iPhone 5s with the app "Indoor Cycling Speedometer with Gyroscpe" in use. During the activity, I have unfortunately been sweating extensively, and now my iPhone has a moisture damage the display.
Conclusion: For less demanding activities, the shell is actually thoroughly recommended, but people with average sweating during intense activities should look for an alternative in order to avoid damage to the iPhone by moisture.