The practical issue that I run into is that I have three kids, and if each got to have what they happened to want at that day and time, I would often be making three different meals for them (not counting the meal I made for dh and myself). I think that a lot of times, what people want at a certain time is kind-of a whim, and our grocery budget and my sanity can't take that. I have tried to teach my kids that having enough food to eat is a blessing and that being fickle is not a good thing. (And by fickle I mean, "Yes, I like this meal, but it's not what I want RIGHT NOW, not "Oh, I hate this meal, why did you make it?")
but i also see thismama's point of view. i think for me it is that i took time to fix something and set out his drink when i could. i might actually not be able to grab something according to his whim later. b/c that usually occurs when baby is having a meltdown, i'm thirsty and have to pee, exhausted and still havent eaten the meal I MADE.