My story's werewolves have beards because they're based on the film The Wolf Man, and that guy was lookin' a little grizzled:
I've never heard of a werewolf with a full beard. People in every era might find a full beard in wolf-form comical. But...it's not WOLVES that have beards, right? Sure, they are completely hairy, but it's goats who are known for their facial hair. Too bad we don't have many weregoats floating around.
Also, as has been mentioned, many modern werewolves (and vampires) are meant to be hunky (think Twilight) and beards aren't always in fashion, especially for teen demographics. (My mom likes the guy who played Wolverine, and he's hairy and almost werewolf-like, but she's like 50.)