There is some validity to the idea of getting less reliable disks for less cost, but it really, really depends on the scale of your systems, and how much time you want to spend doing RAID rebuilds or physically going to the datacentre to replace the hot spares.
I'd rather have assurances that my disks aren't going to eat themselves within a year, or have a failure rate that means I'm going to the DC once a month (And spending money once a month..) to ensure I have enough hot spares to cover the cheaper, more failure prone disks - and that's assuming that the cheaper disks can do fast encrypted writes, if that's a requirement, etc.
I'm pretty sure there'll be a surprisingly big market for these devices - and as more reliable SSDs come on stream (in a datacentre environment, that is - most SSDs are 'good enough' for non-server roles) we'll start seeing them as primary boot drives in standard servers, and then as part of tiered storage, and eventually, they'll take over from spinning rust.
I'll put a fiver on that being within the next ten years, easily....