No reason to not want it. Who cares if pure SSD is "superior?" Sure it is, by about 10%-15%, for whatever is resident on the SSD. But the spinning HDD will always run at spinning HDD speeds - roughly 20% of the speed of the SSD. I'd call 20% grossly inferior.
This person has a choice - a zero-maintenance 7TB Fusion drive running at 80-90% of the speed of an SSD, or a 1TB SSD and a separate 6TB HDD, where the user has to actively manage which materials will benefit from SSD and which don't.
And as to "System files on the SSD, data on HDD..." System requires around 6-7GB. Not many people have 993-994GB of apps. So, what's the point of that whopping big SSD? It'll likely be 20-30% full, 70-80% waste. Once the OS kernel and key modules of an app are loaded into RAM, the SSD would pretty much sit idle.
Meantime, OS and apps are worthless without data. This whole, "data can be slow" thing is total crap. We work with data all day long. So, why should every data read/write run at a fraction of the speed of that SSD? True, there are some data types that are only read once, written never (media files, for example), so they may load no more quickly under Fusion than from pure HDD. But if the same data is hit again? Fusion will have moved it to SSD (with no user intervention), and all subsequent hits will run at SSD speeds.
Now, as far as the question of whether OS X will create a Fusion drive from that setup... I believe that it will, but I don't have the specifics of how to do it in front of me. I suspect that, if you have another use for a 1TB SSD, you may get better value from it in that alternate use. Unless you're using a substantial proportion of that 7TB of storage on a very regular basis, you may detect no performance difference between Fusion with a 250GB SSD and a 1TB SSD. But only you know how you use your system. Huge, frequently-accessed data sets most likely could benefit (let's say if 500GB of the data is accessed frequently), but the kind of data I use... no way.