Most hospital personnel are required to change their gloves between patients. Often they'll strip their gloves on the way out of a room, walk the hallways bare-handed, and put on a new pair when entering a new room. It helps keep infections from spreading past a single room.
Another example, but a rare one, as to why you wouldn't want to solely rely on facial recognition is where say you've got a costume on that covers your face. Rare as I said but you never know.
Does a scarf count? If you live somewhere that gets cold in winter, and walk to get places (for instance, because you live in a city), that's a problem. With Touch ID, I can slip the gloves off my thumb inside a pocket, unlock the phone, put the glove back on, then pull the phone out. Or even use it with just a thumb exposed instead of having to pull the scarf off my entire face and then figure out how to rearrange it properly with thick gloves while juggling a phone.
