If you’re lucky enough to live in a place that essentially has one climate, then you can definitely cut down on what’s needed in your closet. If you’re like me and live in a place like Chicago that goes from roasting and humid to freezing and bitter, then you’re going to need to diversify as the weather changes.
"All-weather" clothing is kind of a subjective term. For instance, the suits, jackets and trousers that would be considered "all-season" wools, but they have some features to them that limit them in the extremes. For instance, they’re not thick enough to hold up against the coldest winds and temperatures, and their full lining (or half lining in the trousers) doesn’t make them ideal for really warm weather, either.
I think the same thing goes for chinos. While on some days I can wear an average-weight chino in the summer, the hottest days just require lightweight chinos. And in really cold weather, I prefer to wear flannel trousers.
Do you need seasonal articles? Not really, I guess. But do they make your life more comfortable? Yup.