Plus, as Relmstein notes, there's added functionality -- the feasts let you lay out an entire meal for your party, and the addition of the Dalaran cooking quests (and the recipe rewards) have turned the daily cooking quests from just a few extra gold and mats per day into a whole currency system on its own. Fooding it up ingame has become a minigame of its own, and with food giving out such diverse (and significant) stat bonuses, not to mention that it often fetches great prices on the AH, there's no reason for your level 80 character to not have worked a little bit on leveling up cooking.
Sold yet? One of the last things I did at level 70 was level up both cooking and fishing using El's Angling's great guide, and though it took me the better part of two weekends in a row, it's paid off many times over since. Cooking has definitely moved from kind of a fun third profession to mess around with to something that will seriously benefit you as a character in quite a few ways.