Leaving toothpaste residue (containing fluoride) on your teeth is the main thing toothpaste is for. Fluoride changes the chemical properties of tooth enamel to

Leaving toothpaste residue (containing fluoride) on your teeth is the main thing toothpaste is for. Fluoride changes the chemical properties of tooth enamel to make it stronger. So removing toothpaste immediately after applying it reduces the effectiveness. If not rinsing is intolerable then rinsing with a fluorinated mouthwash, which provides less fluoride that toothpaste but is still better than nothing, would be a better choice.