A year ago, I updated my OS from 10.6.x to 10.8.x. Rather than running the normal update over my existing system (I have a number of programs that are not compatible with the newer system), I simply bought a new drive, cloned the old system and apps onto it then I did the update.
Since drives also have a limited life span in the real world, perhaps three to six years, I figured that it would be a good idea to have a new drive installed in case the old one would happen to fail since it was close to the top of the bell curve in age.
Now I boot regularly into the new OS yet have the older and bootable system still available for the occasional need to use with the "obsolete" software I still need. I will probably update my 10.8 to Mavericks once I have most of my work complete in a few weeks. Let others be field beta testers for Yosemite!