Steve and Bill F...
The reason for rigging a vertical dipole is simple: there's no other seagoing antenna you can rig on a sailboat which will come even close to its performance. It is a killer DX antenna which will WAY outperform any backstay antenna.
The dipoles I use are "marinized", i.e., made from marine-grade materials designed to stand up to the harshest weather and seaway. Details can be found here:
Gallery :: Constructing a Marine Dipole Antenna
Click twice on each pic for full resolution.
I rig my dipoles on the foredeck, about halfway between the forestay and the mast. They are hoisted with a spare halyard, with the lower ends tied off on the toerails. Vertical dipoles should be rigged as LOW as possible, not as high as possible.
I use short lengths of RG-8X coax to run back to a spinnaker track fitting on the leading edge of the mast, then down the mast to a little black box installed just under the gooseneck. This box is permanently connected to an antenna switch below near the radios.
The dipole or dipoles are deployed in this fashion when in a marina, at anchor, or on very long tacks offshore. To sail, it's only necessary to loosen the lower end(s) and pull them back into the shrouds. They will work there, but will some detuning. The process of setting up for sail takes less than 2 minutes, as does the process of setting them up again at anchor or in a marina.
I don't use running backstays, but if I did it wouldn't matter at all, since my dipoles are rigged on the foredeck not aft. When I first started using dipoles I'd rig them aft, with a long coax lead down the hatch to the radios. But, after awhile I hit on the little black box idea, and it has worked like a charm for 18 years now. Very simple. Very effective. Never a problem when underway.
Bill