Surely a hotspare for RAID1 has more uses that just that. If if you have three disk and use all of them in your RAID1 array then the OS has to write to all three disks all the time.

If you make the third disk a hotspare instead then you won't have to take the overhead of writing to the third disk normally, but if one of the other disk fails a rebuild to the hotspare will start without any intervention.