It's handy, because we're a visual bunch and if you look different from the headshot that GOT YOU the appointment, you could confuse some folks. But I've never seen it be such a huge deal that it costs the actor a callback he/she would've otherwise earned. Generally, if it's an issue, it's a quick, "Oh, you look more like this shot than that shot in person," or some comment like that, and then we move on. :)