It is very good to have a hot spare drive, but the hard drive used as a hotspare ought to be tested every now and then.
Suppose you build your own RAID array and assign N hard drives (including the hot spare disk) to the array. Surprisingly there exists 1/N probability that exactly the disk which is designated to be a hotspare will be the first failed disk. And now you no longer have a hot spare.
If one of the array member disks fails you find out that the hot spare disk has already failed. To avoid this you need to scan a hot spare on a regular basis or stick to RAID6E or RAID 5E/EE layout.