With OS and apps on the fast drive and data on the slow drive, you've made a hard-and-fast decision as to where you need your speed. It may be exactly what you need. Or maybe not, depending on how you use your data and apps. You might be wasting expensive SSD space on apps that barely get used, and wasting time whenever you access a frequently-used data file.
One thing I like about Fusion is that it's adaptive. I'll get the speed where I need it, rather than where I think I'll need it. If I end up needing more space for OS and apps than I have room on the SSD, I won't pay a big penalty when I need to run those off-SSD apps (or spend time re-arranging the locations of those apps). And, since most of my apps are worthless without data, and my real work is contained in the data files, I'm not sure I want to permanently relegate all my data to the slow lane.
Now, Fusion can't possibly be as fast as pure SSD, but all the tests I've seen show that it comes remarkably close. The real benefit, then, is to extend most of the benefits of a high-priced SSD to a much larger, lower-priced data store (up to 3 TB at the moment). I think that's going to give a whole lot of computer users a whole lot of bang for their bucks, for no effort.
In a lot of ways, this situation is no different than the interplay between RAM and disk cache. We've allowed our OSes to manage that for decades, with our only management decision being, "How much RAM can I afford?"
