If your pack is heavy enough, you generally get benefit of having a full waist belt to support a portion of the weight. Once your pack gets light enough, that full waist belt may not serve much purpose. However, I find that a thin waist strap is still useful for minimizing sway of the pack. I have better balance with a thin waist strap that can be tightened.
If I am carrying 30 pounds or more, I use a pack with a full waist belt. For 15-25 pounds, the thin waist strap is sufficient. Only below 15 pounds would I think about going without.