Disks in memory are useful for one major reason - speed. It is invaluable for test execution. While the idea is not new, there was no incentive to explore it until I've run upon tmpfs reference in Ubuntu Wiki.
For example, to get 2Gb of space for files in RAM, edit /etc/fstab to add the following line:
/var/ramspace is now the place to store your files in memory.tmpfs /var/ramspace tmpfs defaults,size=2048M 0 0