In my experience, Wort WANTS to become beer, and there are few absolutes in brewing. You can make fine beer with minimal concern for sanitation, recipe design, mash times, water chemistry, yeast health, fermentation temperatures, carbonation procedures etc. But each point of increased care and attention adds a few percentage points to your chances of making GREAT beer, and reduces by a few points your chances of disappointment. Sure, you can bag the yeast starter, never use O2 or a stir plate, "sanitize" with tap water, and so on, and still do OK; maybe for a few batches, maybe for a bunch. But I think the odds favor those who take the extra effort along the way.