I agree that NVMe is better - numerically, far better. But for standard file access experience, the big difference is between SSD and any form of rotational storage.
The change between, say, a 95% reduction in latency and a 98% reduction is hard to notice; for games you might be talking a second more vs. 20 seconds with a HDD.
This is likely to become more of an issue with DirectStorage, and host-based cache may also make a difference, but overall I think the prices are justified for the experience you get. And at the end of the day, SATA lanes take up less bandwidth so you can fit more of them in, increasing total capacity.
On my own mini-ITX build there are two SATA drive slots on the back and this would go perfectly in them. They're also a great option for replacing that old HDD that's even slower because it's filled to the brim or is starting to show errors, now that they're widely available in equivalent capacities.