While in a perfect world it would be great to have every horse watched for the slightest unusual sign, it's just not realistic. I say that as long as the horses are checked on a regular basis then it is not necessary for a yard to be manned 24/7. Even if someone does live onsite there's no garuntee they would be there if something actually happened. As a previous YO said they could be at the supermarket or a show or a parents evening or any of the other "real life" things that people do. Of course you could have someone employed simply to stay on the yard on the off chance... but I don't see anyone really wanting to pay the livery fees that would attract!!!