Eh, there could be privacy implications. E.g. you see someone in the restaurant whom you know, and you know he is not supposed to be drinking alcohol (for whatever reason: maybe his religion forbids it, maybe there's a medical reason for it such as a prescription drug he's on that really should not be mixed with alcohol, the reason doesn't really matter in this example). You see that he was served a pork chop with a side salad, so you scan through the check numbers and find out that only one order contained a pork chop and a side salad that day, and that order also included a glass of red wine. Congratulations, you have spied on your acquaintance and obtained potential blackmail material on him. What will you do with it? How good or evil a person are you?

And although that's a low-probability scenario, it's also something that could be solved pretty easily, by either using a GUID or at least random numeric IDs with 8 digits.