Etiquette is well and good, but.............?

Today I ran across a classic television show. In the show the family was at the dinner table eating away when Beaver made a comment to Wally that he had his elbows on the table! I don't know why but it struck me as funny. My mother even use to quote "Hamlet, Hamlet strong and able get your elbows off the table. This is not a horses stable but a first class dining table". I don't know if that's the way she said it or not, but for YEARS it has been proper etiquette to not put your elbows on the table when eating. What is it harming? Why is this a rule in the book of etiquette? I've heard many excuses, such as hygiene or invading someones space. In my opinion, putting your elbows on the table is no worse than placing your forearms on the table and you have to put something on that table when you eat your food. And as far as invading someones space goes, move to the other end of the table, and if that's not enough room than that person does not need to be eating at the table with others. I don't get it! Do you?