I think 'worth it' is a relative term depending on how much you have to/want to sit together. If you "absolutely have to" then yes, its worth it. If "it would be nice but not necessary" than save your $$ for some frozen drinks.
My sister gets intense anxiety on planes and its much easier for her to sit with someone she knows so will naturally shell out the extra $$ to do so. While I love my wife, its not that important to us on a short flight so we save our money and roll the dice.
It's up to you.