its all about cost. with ssd caching you can buy a small 20, 30, 40gb ssd together with a nice big hdd and it will super speed your pc. The total cost of this is cheaper than having a single big ssd.
We will be adding the option to select a small ssd cache drive shortly.
I cant really see the point of having such a big ssd as a cache drive. Yo may aswell save money and get a smaller one, or use your 120gb ssd as a stand alone boot drive.