Different people have different opinions. Some say you shouldn't drink a cappuccino after breakfast. Others say not after noon. What's the big deal?
Well, for some people, it's probably not. The average Italian barista is probably used to hordes of tourists asking for one at any and all hours of the day. Even I used to order them for my afternoon dose of caffeine, that is, before I fully understood the logic.
Say what you want about Italian government, culture, whatever. But one thing you have to begrudgingly accept after spending time in Italy is that the Italians, when it comes to food, know exactly what they're doing. Culinary quirks that seem strange when first encountered make complete sense once you stick around long enough to be able to understand them.
Take, for example, eating salad after, and not before, a meal. At first you may think it's just yet another instance of different traditions and a different way of doing things. I know I did. But at a certain point you realize that both choices are not equal: it makes so much more sense to eat the salad after the meal. This way the salad is a cool, refreshing end to the meal and leaves you feeling refreshed, not heavy. But regardless of how many people may tell you this, you won't fully understand it until you experience it yourself and think for yourself 'OK, this does make sense'. (Did I mention that you become a food snob in Italy? You really can't help it though, as everything is so good...)
So coming back to cappuccino, the reason you don't order it later in the day is because it contains so much milk, a beverage Italians don't consume nearly as much of in comparison with their American counterparts, and which, as any informed and educated Italian will tell you, is just too much to drink on an empty stomach late in the day. The very thought of doing so will make them (and those of us who have seen the light) cringe.
Of course, all rules were made to be broken. For example, if you're going to meet someone for coffee and know you'll be sitting around talking for an hour, order a cappuccino. This way you'll have a drink that will last a bit longer as you sit down and chat, as opposed to an espresso, which would disappear after a few sips.
Like with the English language, the rules are often broken... you're just supposed to understand them first.
