Re: You missed the point
No you can't. At least not in my experience. I've already put a hybrid drive in my laptop which uses SSD as a cache (a Momentus XT), and it performs nothing like my desktop, where I have a separate SSD for the system disk and use 2TB HDDs for data. Yes, it's better than a normal laptop drive, but it still struggles with things like system updates. Admittedly, the hybrid drives have nothing like 128GB of flash, but I, personally, would like to manage what is placed where. Cache algorithms are all very well, but they tend to fail where there are periods of intensive activity (such as system updates or software installs) which are relatively infrequent, yet cripplingly slow when they occur.
Also, if you assign all that 128GB in a "traditional" cache arrangement (as the hybrid drives do), then you lose that much space. If you go for a more sophisticated approach to avoid that loss of space (which effectively migrates data seamlessly between slow and fast store), then you have to put up with the overhead and the unpredictability.
Of course separating your data and OS is an essential of this approach, but frankly it's good practice to do so, In fact, when I get a new machine, my first approach is always to re-partition to get functional separation. That makes it much, much easier to run robust backup and restore regimes. That separation is good practice, whether you are running a massive server or a laptop.