Yeah, I've got an SSD for OS and programs, and it seems to me the best use. I haven't experienced a hard drive streaming bottleneck in about 10 years or more, so it's not a problem that needs solving as far as I can see.
On the other hand, the experience of booting up fast, loading Sonar fast, launching plugins fast, passing audio in and out of external editors fast; that's a tangible, and sizeable, improvement in your working day.
I want to go fully SSD eventually, as the hard drives are the last part of my computer that isn't completely silent. But it's not a pressing need; going to wait for prices to drop some more.