As an interim measure this would be a good viable option. Mixing technologies as you propose should be able to give SSD performance, larger capacities and cheaper prices, however as SSD’s become more mainstream and prices drop to HDD levels and capacities increase the value proposition would quickly disappear. I guess the question is how long would it be a viable option and how much would it cost to develop?

You would have thought that Western Digital would have considered this, but then again it’s amazing how big companies are disjointed when it comes to combining technologies. (Raid trim support and Intel come to mind quite quickly).