Not a stupid question at all. Yorkies weren't bred for the extreme cold, so you really should take precautions. With the exception of a short trip out back to pee, I put a coat and booties on mine any time it's lower than 30F. We go on walks down to -5. Any colder than that, I play with them in the house for exercise and let them out briefly on a shoveled area out back.
If the cold doesn't seem to bother them, I don't think they HAVE to have booties for short walks. Dogs can get frostbite, but generally not in 10-15 minutes, especially when they're moving. Salt on the road or driveways is another good argument for booties.
Mine don't especially like booties, but they can handle longer walks with them on. One of Eddie's fell off this weekend when it was about 30, and he immediately started lifting up that foot, telling me it was time to go home. Jillie will walk forever with booties, but she wants to end walks quickly without them.
The best booties I've found are simple fleece sacks, like socks. They're so light, the dogs learn to tolerate them quickly. The secret to the ones I buy is that the velcro is attached to an elastic strap at the top. The elastic allows you to get it snug without cutting off the circulation. I buy them from a woman in town who patterns them after the ones used by sled dogs, but I'm sure they could be made pretty simply.
The woman I buy them from has a website,
Page 1, but her pictures aren't very good. Also, you have to e-mail her through the Contact Us link to order. I buy 3XS for 4-pound Jillie and 2Xs for 10-pound Eddie.
Hope this helps.