http://nex7.blogspot.co.uk/2013/03/readme1st.html Scroll down to #12.
The problem with hot spares is that you could be removing single points of failure. For instance, if you have mirrors, each with a drive in two different JBOD's, and you lose a drive in JBOD A and Solaris/Omni/Etc chooses to use a hotspare from JBOD A, you now have a single point of failure.
Some general guidelines from the blog:
The idea, of course, is to determine if hot spares are seemingly
required, or if warm spares would do, or if cold spares are
acceptable. Here's the ruleset in my head that I use after they tell
me the answers to that question (and obviously, this is just my
opinion on the numbers to use):
Under 24 hours for remote access, but physical access or lack of disks
could mean physical replacement takes longer Warm spares
Under 24 hours for remote access, and physical access with replacement
disks is available by that point as well Pool is 2-way mirror or
raidz1 vdevs Warm spares
Pool is >2-way mirror or raidz2-3 vdevs Cold spares
Over 24 hours for remote or physical access Hot spares start to become
a potential risk worth taking, but serious discussion about best
practices and risks has to be had - often is it's 48-72 hours as the
timeline, warm or cold spares may still make sense depending on pool
layout; > 72 hours to replace is generally where hot spares become
something of a requirement to cover those situations where they help,
but at that point a discussion needs to be had on customer environment
that there's a > 72 hour window where a replacement disk isn't
available