By far not the only advantage, and by far not the most important.
The real reason is this: The way that Fusion works, every bit of data has exactly one spot on the combined hard drives where it is supposed to be. The exact spot can change over time, but it's always one place. As a result, if things go wrong, your computer crashes, or power goes down, everything is always where it belongs, minimising the potential damage.
With a pure read cache, the same would happen, but 128 GB read cache only would be idiotic because it doesn't accelerate writes. With a 128 GB read / write cache, everything that is in the cache now has two locations where it should be: One in the cache, and one on the hard drive. You always have substantial amounts of data that is not in the place where it belongs. So when things go wrong, you're in trouble.
Add full disk encryption to the picture, and using 128 GB as a read/write cache is something that I personally wouldn't want to have to implement. What Apple did is very simple, very effective, and very easy to get right.