My dive parnter and I use one in a very specific situation. There is a cove at a local lake where boats gather and anchor, particularly on holidays. The water is fairly shallow (about 30 feet) and chronically murky (vis about 3 feet). Subsequently, partiers drop quite a few items, and my buddy and I scavenge for "treasure" once in a while after a busy weekend, just for fun. We use a tether on these forays to keep from getting seperated. However, there are a couple of things that need to be noted:
1.The tether is about eight feet long, with velcro bands on the ends that go around our wrists, and we have two small floats to arch the tether and keep it off the bottom. Entanglement is minimized, and dragging the tether is avoided. Getting rid of the tether is very easy should there be any problem.
2. As noted before, this is at a lake. There is no coral, or current for that matter. Again, entanglement is a remote possibility.
3. The only use for the tether is to keep us from getting seperated while our attention is focused on the bottom while we are scouring for stuff or when we are retrieving something. That never fails to stir up clouds of silt, and losing eye contact would be a virtual certainty. While cruising, we still attempt to maintain visual contact, of course, and we check with each other often, even while using the tether. It's a tool...not a crutch.