For a shelter, I would use cord. However, zip ties are very usefull to have. On a recent backpacking trip, one of our group members hip belt quick disconnect split lengthwise, preventing the buckle from latching. Para cord would not have helped. A zip tie pinched the break together well enough that it lasted 8 days with a 50 lb pack. I usually carry a few in a pack repair kit with a couple of pins, split rings, duct tape, & needle and nylon thread.