"Should" is such a strong word.
Looking to the evidence base isn't much help either as there's plenty of evidence to support pretty much each and every eating style, modality, and diet. Moreover, I'd argue that even if the evidence did firmly fall into the camp of one style or diet being "the best", that wouldn't change the fact that following it may prove non-enjoyable (and hence non-sustainable) to many.
"Should" presumes that everyone is the same. Same genetics, same co-morbidities, same lifestyles, same likes, same dislikes, same, same, same.
The thing is we're not all the same.
In my practice we tend to start people on 3 meals and 2-3 snacks a day, all inclusive of protein and at least 300 calories a meal and 100 a snack. We do that because experientially with us, for a great many folks, that spread has proven helpful in reducing cravings, hunger and struggle, and for busy people with jobs and young families, it's often more practical than larger, less frequent meals.
But it's not useful for everyone!
We have other folks on 3 square meal a day regimes.
I've even suggested an intermittent fasting (IF) style 2 meals a day to folks whose lifestyles and struggles suggested they may be well suited to it.
And as far as dietary style goes we've had Paleo folks, classic low-carb'ers, vegetarians, vegans, low-fat fanatics and certainly everything in between.
In my clinical practice, I'm not married to anything other than a person living the healthiest life they can honestly enjoy.
Those folks who firmly believe everyone "should" be doing things a certain way? That there's one "right" way to diet; one "right" way to eat; one "right" form of exercise; one "right" road to health?
I think they're wrong.