There is a tangible advantage to doing this.

There is a great deal of misconception, at least in my opinion, about how to improve loading/zoning performance in wow. A lot of people seem to think that spreading 5 installs across 5 disks would be the epitome of configurations since you can have a drive per installation. The problems are: a) you will generally access these in a serial fashion, losing much of the advantage that could be gained (parallelism), b) you lose per-disk caching, and c) you lose system level disk caching.

Emperical results: I saw a significant performance increase when I went from 5 directories to 2 (want a separate slave dir for graphics/performance settings) and symlinked everything but per-account settings and mods.

I do have a SSD, and I movec everything that I didn't want in common for ALL my wow installations to it. You don't, in general, want to put stuff on there that gets written to with any frequency because it is slower, and writes are largely what "wears" a SSD.

My advice: move to the smallest number of install directories that you can to support what you want and symlink the /data and other mostly static files to an SSD.