"I've seen this advice many times, and I just don't buy it. At home I eat the same breakfast every day, and it carries over to the trail - the only change is olive oil rather than butter for my bread. Lunch is nearly identical every day as well. Having found things I like, I keep liking them."
Yeah, this is more or less what I do, too.
For long trips, it is always coffee/cocoa, vitamin pill, and oatmeal for breakfast and maybe a wrap at lunch. I rarely eat much of a lunch, just some crackers over the course of a day. Suppers change, though. Stews, soups, rice, noodles, with or without peperoni, dried beef, salami, dried sausage, jerky, fish, etc. Always cocoa and a shot of scotch after the meal. Parified butter in the oatmeal and olive oil in my supper. Still, over 10 years of packing this you might think it gets old, but at the end of a 20 mile day it ALWAYS tastes good!