I have it as my system drive... and my old 2 x 500GB HDD in Raid0 have been relegated to general use.
Before the SSD, my system went on, and HDD would be active all day, as the OS is always messing about doing something.
Now, for first time I notice the HDD spin up when I do something that needs them (If I just Inet browsing etc - looks like it all done via SSD)
We know that in general, start up is when most strain gets put on devices, so... out of curiosity - you think that I will be shortening the workable life of my old HDD by shuffing all OS activity to a SSD and putting old HDD in situation where they spin up and down a lot more?
While some folks speculate that the HDD part of a hybrid drive may not last as long due to the increased number of spin ups and downs, I don't know that there are any good studies to prove that theory.
The whole idea behind green drives is to spin down when not in use and I've not personally noted any decreased life yet on dozens of drives over the last 3 or so years since I started using them.
Besides the awesome performance increase is worth buying a new HDD sooner (if it really happens that way), just back up your data as you should anyway.
I wouldn't worry about it. In the old days stopping and starting a hard drive caused head wear because when the drive stops spinning the heads were no longer held above the platters due to air pressure. But modern drives use head offloading ramps and when they spin down the heads are moved beyond the edge of the platters - so they never touch the platters even when the drive is stopped. That change has eliminated most of the lifetime issues associated with hard drive stop/start cycles.
As an example, WD rates their current drives as being able to sustain 300,000 start/stop cycles. Even if your drive stops and starts 100 times each day, that still works out to about 8 years, which is pretty much "doddering old age" in disk years.