I agree with this - darts can be useful - they shouldn't be a replacement for taking in the side seams but can be helpful in the correct circumstance.There's nothing wrong with adding darts to the back of a shirt, if that's what the fit needs. They can achieve more waist-suppression than the side seams alone if placed near the side seams (like my bespoke shirtmaker does), or they can be placed further in to take in the shirt in the small of the back. Side seams alone can only achieve so much waist suppression before the shape of the shirt is distorted, and they cannot do anything about too much fullness in the small of the back.
But you are also right that darts are often used as a shortcut. They should only be used if the shirt already fits fairly well because they can only have a small effect on the fit.