Fishing in contact with the bottom certainly leads to a higher success rate than fishing just below the water's surface, but if you can guess the right moment when trout and grayling are concentrated on eating insects in the ascending phase, while they detach from the bottom to reach the water's surface, then having slightly weighed down the nymphs with glass beads will really make the difference. In fact, in certain situations, the weight of tungsten is much lower and even slightly lower than brass will allow you to be very effective against both heavy nymphs and the classic dry fly.