The arrival of the Great Feast and Fish Feast in Wrath of the Lich King freed up much of the burden upon guild chefs and eased many a headache of raid leaders attempting to ensure every person is food-buffed. Flasks have been around for a long time, providing a monetarily advantageous way to buff yourself for a raid without needing too many elixirs or having to renew them each time you died. However, these methods are not always the best choice, and actively work against min-maxing characters.
Fish Feasts are an easy catch-all. With attack power, stamina and spellpower, everyone in the raid has something to gain from eating Fish Feast. But that does not mean it's what they should be eating. Food that increases hit, crit or haste may be more appropriate, or strength food may give an edge over straight attack power. Expertise food, or perhaps mp5, could also be utilized. But this becomes largely up to the individual, and if they do not have (or do not wish to spare) the funds, or do not want to min-max, it's possible to lose that boost.
There was once a great multitude of Flask choices, but with the introduction of Spellpower and no base stat booster flask, the choices have dwindled down to four: attack power, health, spell power, and mp5. Much like the Feast, they're convenient and homogenized so every class and spec has a generic flask they can choke down and pewpew with.
But with two elixirs giving more of a varied boost, they allow for greater min-maxing of your character. If you need hit, or crit, or perhaps a little bit of a larger mana pool, or some health and expertise, nothing works quite as well as dual elixirs. But it is expensive, needing refreshers after each death as opposed to after a certain time limit. As well, doubling up elixirs may provide with a more even benefit, but they do not provide as much in a single category as a flask, with less spellpower or attack power.
Overall, it is a little up in the air. This is not to devalue the Flask or the Feast. They are intensely helpful, and should always be utilized. But knowing what other choices you have, and that the flask or feast could possibly not be the best (or a good) choice, is valuable information. When your raid wipes at 2% on Deconstructor because he hit enrage, that little bit extra could have made the difference.