Nothing wrong with Caching.
HyperOs do a complete ram drive.
So does gigabyte (http://www.youtube.com/watch?v=Gp676uX3EXA)
Both of these has battery backup. The HyperOs actually allows does a system which backs up automatically to SSD in a power failure.
These two alternatives redefine performance for me.
I see no problem at all with having a tiny rechargable battery in an SSD, to provide a minute's power, if the solution allows this kind of performance for writes.
I've long since thought that DVD drives, and Harddrives should have their own Dimm slots so users can buy cache ram. A DVD ram with a 8 GB ram would transfer the whole hour of music into cache in 90 seconds, and from then on not spin up again, likewise an inserted DVD would take 7 minutes of spinning, after which it would provide a random access seek of basically nothing.
Similarly, with very little work, and 16GB of ram on a hard drive, Windows XP's read only files, such as C:\Windows and C:\Program Files could be cached on first use, or on lazy read. They almost never change. You could even cache files, just on the basis of the head passing over a track on the basis that "We're going to track 5 sector 3 to get something. We might as well cache most of Outlook.exe on the way, as we're passing over it to get there."
I'm surprised, bearing in mind the cost of Ram, that Microsoft hasn't got huge ramdisk mirroring of files that aren't modified. It's obvious.