I'd buy the strap because otherwise I think they look stupid. I honestly think the strap makes them look better plus if I want to take them out for a while it adds the convenience of just hanging them around your neck.
Have none of these "Airpod leash" developers taken the time to understand how the product works? Not only will the weight and movement of the cable almost certinly cause them to fall out much more often it will also prevent the quick and seamless recharging using the included case.
If you don't want two independent ear buds then you have a plethora of choices in headphones. Why someone would try to shoehorn a different product into working instead of using the correct one in the first place is entirely beyond me.
Totally defeats the purpose and design of these headphones. If you need to buy this cord then you're better off looking for another pair of BT headphones on the market that surely sound better than the airpods