There are a few questions/points at issue, but I suppose the fundamental one is when do you, or when should you, wear a life jacket and/or a tether on the boat as a matter of routine? What are the risks?: 1) falling overboard, 2) being knocked overboard by something, or 3) being washed overboard by boarding seas. Crew holding-on (i.e., one hand for boat) while moving about presents a low risk of falling in unless not holding tightly enough; where one trips and still falls in. You could do this all day long over thousands of sea miles without experiencing much risk. However, a tether would be a good idea here if you need both hands for something and cannot hold on all of the time, as in a sail change. Getting knocked hard by something like a wild clew cringle, swinging spinnaker pole, the boom during an unexpected jibe, or even another crew member, might put you into the water dazed or semi-conscious; again, if not holding on tightly at the time. With boarding seas, I know people who have literally been washed out of the cockpit either over, or though, the life lines and into the ocean.
At minimum, IMHO, a solo sailor sailing in heavy seas should be tethered even while in the cockpit. This is preferable to having a lifejacket on only b/c, as someone mentioned, being overboard with the boat "sailing away" is not wonderful either. The risk of being knocked overboard is the most worrisome, however, b/c it could come at anytime, just about anywhere on the boat, when you otherwise might feel secure. Consequently, the only defense is to either be tethered at all times, or have on a lifejacket at all times in case you do go into the water. If you're sailing solo on the high seas w/o a PLD attached to you and you hit the water you're probably toast; if you have a PLD you're still going to need one of those bulky, Type I, offshore jackets to await rescue--the kind that nobody wants to wear around the boat, etc.
My compromise solution, although I don't cruise solo on the high seas as some do so I might be "all wet" here, is to wear a Type III working vest when moving about the boat untethered; in particular, the one shown below. I have one each for my wife and me with our names on 'em. They're easy to put on and take off, light, and not uncomfortable. They usually reside under the dodger foward of the rope clutches when we're at sea. Put it on, go forward or where ever, then take it off when back in the cockpit. They have pockets big enough to hold my hand-held GPS/DSC VHF radio, a knife, light, whistle, and a couple of other items if needed. These are also the ones we use when in the dinghy. Of couse, we have two Type I offshore jackets aboard as well which we would don if conditions got threatening. So, if sailing or motoring alone and I leave the cockpit with the boat under autopilot, I have on the vest and typically my harness as well, with the tether usually attached to the harness at one end with the other end stuffed in a pocket of the vest or of my jeans. If I need both hands for something, I can hook onto the jacklines, or padeye, or something. Nowadays, I keep the GPS/DSC/VHF with me but have no PLD. If I do hit the water still conscious wearing the Type III, and can hit the distress button, I can at least hope for a "speedy recovery" as the boat sails away to some distant shore!
