Reasons for not having a a (fully enclosed) chain cover:
- adds extra weight
- increases wind resistance
- costs more than replacing a worn "standard" chain
- can not fully enclose a derailleur chain + drive train
Reasons for having an enclosed chain cover:
- extends chain's lifetime
- allows for lubing chain excessively with heavy oil, while keeping rider's ankles (socks) clean
- less frequent chain cleaning and lubing
One can install a chain guard that covers only the top and front part of a chain on a derailleur commuter bike. It will protect fairly well against dirt and water projected by the front wheel.