For some the Zone System, personal EI, measuring, calibrating, etc. is a total waste of time when they have their own methods that result in negatives that are exactly as required without all the extra faffing about. They don't require the zone system to be able to make a good negative.
For others, the zone system, personal EI, measuring calibrating, etc. is totally worth the time and effort, makes their photography more enjoyable and is central to their craft.
Both views (and all in-between) are perfectly valid. It's your film, shoot it how you like.