Why protein quality is of little importance in cattle feeding?

Because it's not the nutrient that is needed for weight gain. Energy content of a feedstuff is what makes the animal gain weight: the higher the energy content and lower the crude fibre, the more the animal will gain.

However, protein is still of a lot of importance, because that's the nutrient that makes the animal grow muscle. This is very important for growing calves and lactating cows, as well as for bulls that have just been pulled off of breeding cows and need to gain weight again.

When analyzing a feedstuff, you can't just focus on one nutrient value. All the nutrients, being water, protein, energy, fibre, vitamins and minerals play an important role in the health and growth of livestock.