Friends of mine (a couple) who travel to Europe in coach, always buy three seats so that they're guaranteed an empty seat between them to spread out and insure that they'll not be elbow to elbow.
They're not FF's and can't afford to buy Biz Class so they feel it's a decent alternative. Their one complaint is that they don't get the miles for that seat.
though they do like getting that one extra meal in case one of them doesn't like what they ordered. ^

My guess is that if the airlines gave people the FF miles for that empty seat, a lot more people would be doing it which could bring in some extra dollars and make for happier FA's due to less passengers to deal with.