Imagine some of the lyrics to the Broadway tune "Hair" as a description of beards instead of tresses: "curly, fuzzy, snaggy, shaggy, ratty, matty, oily, greasy, fleecy, shining, gleaming, streaming, flaxen, waxen..." That pretty much describes the worst fake beards from films and television. They're either so awful that it's obvious they're false, or so perfectly plastered on that they look, well, like they've been drawn with a Sharpie.
Some of the world's most beloved, handsome, and talented actors have been the unfortunate bearers of bad movie beards, from Martin Sheen (Gettysburg) to Dwayne "The Rock" Johnson (Hercules). Perhaps the makeup and hair department just wasn't skilled at creating false facial hair. Whatever the case, it's hard to stop staring - or wincing - at a fake beard, which makes the character a little or a lot less believable.
A note to bearded men everywhere, in movies or real life, with fake or real follicles: a chin strap-style beard is rarely a good look. John Travolta in Killing Season can attest to this.