The point (to me) of Fusion is that it is self-managing. Whatever benefits from being on SSD is present in SSD. Just those parts of the OS and apps that you actually use, plus whatever data you happen to be working with.
For my money, placing the entire OS and all your apps on SSD is a waste of expensive space - much of that code will rarely, if ever, be used, and certainly not concurrently. And since you can't fit all your data onto the SSD, you're left manually transferring data from and to the HDD - not exactly an efficient use of time.
Who cares if it's a 5400 RPM or 7200 RPM HDD? The Fusion setup minimizes the impact. Fusion consistently benchmarks at 80-90% the speed of pure SSD - with a 5400 RPM drive. Any benefit you might get from a somewhat faster external HDD is likely to be lost to the inefficiencies of operating without active Fusion management.
The Fusion Drive can be a fine choice for some, and for some users, the idea of having to deal with more than one "drive" is just too confusing to make up for the performance differences. But you're making it sound way more time consuming management involvement there is.
For most users who need more than 250GB of drive space, it's because they have a huge library of something... a photo library, iTunes library, or video editing, etc., and in those scenarios that cover 90% of users who need more than 250GB, it's very easy to keep those on a separate drive - it's pretty much self-managing through the software.
Meanwhile, having the entire OS and all your apps on an SSD is the single most significant performance boost you can give a computer.
It's unfortunate the whole Fusion Drive offers "
80-90% the speed of a pure SSD" has spread around these forums, because it's a gross over-simplification of the trade off. The issue is that when the SSD portion of a fusion drive is working in your favor, it offers 100% of "pure" SSD performance... but when it's not, it's offers 0% of "pure" SSD performance. So what "80-90%" really means is that if you're a "typical" user, you're getting SSD performance 80-90% of the time... and the other 10-20% of the time you're getting HDD performance (which is even worse when it's a 5.4K drive). That may be perfectly acceptable to a lot of users.