A beautiful picture is sometimes better than words :
With standard HDDs (here in RAID 10), the innodb_io_capacity variable may have non expected effects.
You can see on this picture the result of a test on one of my server with only the replication thread activated. The value of the innodb_io_capacity variable was the single modification during the period.
So, don’t touch this parameter without a strong reason…
[EDIT1 2013-01-08]: I know 30k is a too high value for innodb_io_capacity, I used this value to make the graphe really eloquent.
Read this excellent post from Chris for more details : http://www.chriscalender.com/?p=201
[EDIT2 2014-09-12]: Tuning this variable is of course needful in some cases. Do your own tests