Here's my idea:
I'm a writer, editor, and an amateur photographer. So the heaviest software I would use is Photoshop. I think I simply don't need the speed of pure SSD. Fusion Drive might be fine for me.
I have a 4-bay NAS, too. But I believe that internal storage is better than anything external. I want to keep my core data and library simply staying on my main computer. Keep things simple. So 512GB is too small, and 1TB is too expensive.
And the most important thing is, I have never really used Fusion Drive. I can't tell whether it's a good choice for me or not. And I would never know until using it by myself. In the meantime, I think Fusion Drive makes a good balance between storage and speed. And it's much more affordable than SSD.
So I think I can gamble on it. If it suits my computing style, then I would have made a very, very smart choice. If it doesn't, at least I have tried this kind of HD, which is an amazing technology these years. And It's just a computer, I won't keep it forever, I will change it in 5 or 8 years anyway. I will go for SSD without hesitate (if there will still be a choice) and, I can definitely afford 1 TB or even larger SSD at that time.
Also, if Fusion Drive is really a bad idea, I can still change the HD into SSD after 1 or 2 years. Besides SSD itself, the only price I would pay is losing my Apple Care.