Just bought a Vancouver 27 cutter and discovered two extra 'dangly bits' on the mast ! Seriously though, would appreciate some guidance on how and when to use them.
Running backstays are to tension the equivalent height forestay, to stop the mast bending forward and the forestay sagging. On all boats with runners this will give a better shape to the relevant foresail when beating as it stops the stay and hence the sail dropping to leeward and becoming too full.
Also on some rigs with runners it has the additional minor task of holding the mast up. This is the case on single foresail fractional rigs like X99's where the shrouds are barely swept back, if at all, and the backstay is more of a trim control than a mast support. One runner must be on at all times when a sail is drawing, which causes no end of fun when trying to gybe. Some pretty deft winch work is required.
For your rig I am in no doubt that your runners server the former purpose of tensioning the inner forestay. Whether they are also required to hold up the mast as well, I would guess not given the type of boat, but this needs confirming by a cutter sailor. Any J class skippers out there?
I've got a Vancouver 28 which is rigged the same. The running backstays are really there to tension up the inner forestay. The mast is pretty well stayed without them although it's quite comforting to know you have a couple of extra stays should you need them. When beating, they make quite a lot of difference in how tight you can get the staysail.
Unless I've got a least one reef in the main, I don't bother with the running backstays ( I was told this by Northshore ). I run them back to the shrouds and leave them tied up there.
When you have two reefs in, they don't get in the way as the main in underneath them. That just leaves the times with one reef in when they're an extra thing to slack off or tighten everytime you tack or gybe. Not perfect but not too hard to live with.
The running back stays are absolutley esential to use when you have the staysail up. Not only will it apply the necessary tension to the luff of the staysail, it will stop your mast breaking from the pull of the forestay. If you are using the staysail in any decent wind - you risk loosing the mast of you don't tension one of the back stays. Just imagin the forces operating at the level of the inner forestay and see how it pulls that part of the mast forward. If there was not runner, the mast would fold up fore and aft.