A 2008 Sun article sees Flash as having the following position in the server storage hierarchy: cache, DRAM, Flash, disk, tape: http://blogs.sun.com/jonathan/entry/not_a_flash_in_the

Which is what the present article is talking about in this line:

The benefits of SSDs as an intermediate caching tier are also limited, and the cost of provisioning such a tier was justified for fewer than 10% of the examined workloads