No reason not to wear one 100% of the time, tying off before your feet leave the ground and detach only after you get back down.
Any reason for not tying off 100% is bogus -- the technology is there and my life is worth more than the $100 it costs at a local sporting goods store. People gamble their livelihoods by taking such unecessary risk.
I certainly don't want to rely on the aflac duck paying my bills for the next 18 months while I go through physical therapy and hope my employer holds my job open until I can return. If I'm alive to return that is. "Lets see, on disability, x-mas is coming, can't work but may go back next year, but I'm alive, yet not enough $$ for x-mas, spouse had to get another job, making minimum payments, interest accruing is staggering...etc"
Foolishness. Plain and simple. People who elect not to tie off need to think about the day after they fall.