I use them. Basically the nocks on the last batch of arrows I made suck and tend to slip. Using one on top and one on bottom stops the nock of the arrow from slipping up the string. I like it, it also can help keep you from pinching the nock of the arrow when you draw.
EDIT: I actually have 2 on bottom because my nocking point was off and I couldn't get the nock off the string so I just put another one right above it and it has worked just fine. So actually I have 2 bottom one top.