Cold is a relative concept. I live in Southern California. Our weather during late fall, winter and early spring is just a constant heatwave to much of the world. For us, cool weather starts in late September and our seriously cold weather begins in late December and usually lasts through March and April.
If the weather and temperature is relatively constant throughout your ride you don't have to wear layers that you're going shed, so the convenience of arm warmers versus a long sleeve base layer isn't an issue. Our weekday morning rides are generally 25 miles and begin at 6:00AM and we're usually back around 7:15. The temperature is relatively constant during these short workouts.
At any temperature you should wear at least two layers: Base Layer and Outer Layer. The Outer Layer is usually your cycling jersey. The Base Layer is the most important one. When it's seriously cold (36F - 49F) I wear three layers and a skull cap and booties. The middle layer provides the real warmth and comfort. The skull cap is a critical component. If your head stays warm, it's easier for the rest of your body to stay warm, especially your hands and feet. If you need extra warmth or a wind break to block wind chill you can wear a vest over your jersy.
Cold Weather Clothing Components: