Who says they all need to be on the deck? I installed a pair of vertical lines to deflect the genoa sheets away from the mast-mounted halyard winches (one on each side), around which they loved to foul during tacks. They are anchored to strong points and the line is 1/4-inch Dyneema, selected because I had it and it was non-stretch. At first I was concerned that the deflectors would be in the way, but I soon realized that they were handy holds and clipping points when working at the mast, much better than clipping to the mast base. I can even lean on the tether in rough weather, allowing for better 2-hand work.
Jacklines and Lifelines as Hand-Holds
It is often said that using the lifelines as handholds is a bad habit. While there is truth to this on a monohull (the leverage on the stanchion bases is cruel when hauling to one side from the deck of a leaner), I disagree for catamarans. The difference is that cats do not heel and that most of the motion is vertical. In heavy going cats pitch, and sailors near the bow can be nearly weightless. Additionally, my jacklines are relatively high at the beam, since they are secured to the hard top. Thus, the safest way to traverse the side decks between the hard top and the tramp is to hold the lifeline in one hand, the jackline in the other, and pull up. Not to the side, not push down, but pull straight up such that your feet are held firmly on the deck. There is little bending stress on the stanchions because the load is vertical, and the support is steady rather than surging roughly as the boat moves. Intuitive to a lifelong climber, something may lubbers and sailors have to be shown.
Pulling up (not to the side!) on lifelines may be a little hard on them, but it is easier on me. Not a mistake, but a well-reasoned standard practice.
Probably just a catamaran thing.