Say a casino has 5/6 deck pen games. If you're at one table playing like you normally would, why wouldn't you want to keep tabs on the table next to you, even if it's a 4.5/6 or 4/6 pen game (effectively, with missing cards)? No, it's not going to be perfect....but it doesn't need to be perfect. I'd rather have that information than not, even if it's a little off.