A very bad scenario for a windows admin is when users cannot login in the morning because the DC is down and they have nothing to do except of calling in and asking when the problem is solved. This is the reason why admins like a second DC to just have logins working if the first DC fails.
So the little advantage to hotsync in this case is the automatic failover, possible because of the AD structure serving different logon servers.
You are right, it would make much more sense to have a full backup of all services, not only AD DC but it’s a single point of failure affecting all users and windows admins are used to it.
What about combining hotsync with read-only DC or make them work on the same server?
Could the read-only DC be a starting point to an AD migration scenario?