I think that arm warmers are too bulky to carry when not in use, move around too much when they are on for me to be comfortable with, and only have limited use.
I prefer a polyester shell bicycle jacket. It balls up smaller and lighter than arm warmers (easily fits in jersey pocket), is more comfortable, and is much more versatile. Leave it unzipped, and it warms and cuts the wind along your arms. Zip it up if it gets colder or starts raining. If it is a size too big, you can wear more layers underneath for when it drops below freezing.