I ride a motorcycle year long in Paris, it's definitely not comfortable during the harshest winter days but with proper gear it's definitely doable. Spring and Autumn is fine, you just need waterproof gear for rainy days. At lower speeds and with exercise keeping you warm riding a bicycle is probably even less of an issue.

Sure, on rainy or super cold days it's not like it's as comfortable as a car but you'd have to pay me to drive a car around Paris or any big european city at rush hour.