It's your money that you are putting into the game, the decision is yours whether you actually buy it. I myself only buy ranks if i have money spare that i don't have a use for, otherwise i'm not going to put money in this game.
Like MrfatsoJr said about your money. I always buy ranks since I feel they are worth it with the benefits. If you're going to invest time into the game, why not improve your experience? Keys typically aren't worth it since they only last the season and you won't be given them after any resets. If you are set about buying something, I'd go ahead and grab a rank