Hey Del,
Jon lines come in handy for lots of reasons. As you saw, they reduce clutter at safety stops and deco stops. In rougher seas they can significantly reduce the jerking and stress on your body as the anchor line jerks up and down - you're offset off the line by some 6'-7' and you consequently move much less 'up and down'. In an emergency they can secure two divers together on the surface, assist with recovery of items using liftbags, and many other uses.
Simplest are the ones that look like dog leashes. Make'em yourself from flat nylon tubing, available at many outdoor stores (REI, EMS, etc.) and a large brass clip. (Less than $10, plan on dropping them from time to time. You'll need at least 9'-10' to tie the knots and leave yourself a pretty big loop.) Flip the loop around the anchor line, run the clip through the loop, hang on to it. To move up, move to the line and loosen the bight. Pretty simple.
Another option uses a clip like this one (scroll down to 'jon line clip'):
http://www.nextag.com/AQUA-EXPLORERS-Scuba--zz2701197zAQUA+EXPLORERSzB10z5---html
To use, just fasten this clip over the anchor line.
I bunch mine up and hold it with a rubber band made from a cut up bicycle innertube. It clips into one of my thigh pockets.
The standard admonition is to never clip yourself to anything underwater. Hold onto it in your hand so you can let go and jettison in an emergency.
In practice, for gas switches both hands are nice. Photogs often like both hands to secure camera gear, etc. On a long deco stop I clip mine to my scooter, which is in turn clipped to my crotch D-ring. So, lots of guys clip off and relax - just make sure that in the event of sudden bozonity you can cut it free.
Finally, this subject has been covered often, and there may be some other links, input, and worthwhile comments. If you do a search (command line, top of the page, third button from the right) you may want to peruse a few of the other threads on this topic.
Hope this helps.
Doc
