The answer is simple, and has been stated repeatedly, but I don't mind posting it again: Money. (and to a lesser extent, time.) Given sufficient money, it's a no-brainer that pure SSD in a PCI slot is the best performer. That sort of thing isn't always in the budget. For someone cost constrained, or someone trying to make best use of existing bits and pieces, the Fusion drive is a reasonable attempt to make the best of things. It will work really well in some usage scenarios and will work poorly in others. In those cases where the Fusion algorithms don't work out well, one can try manually separating files into SSD files and HD files, but that costs time and effort which a user may not care to expend.