[xandrius]

Sure, if you're going to reuse something which would be thrown away or left to dust otherwise (foolish but I'd imagine someone does that).

But don't do this just so you can upgrade your current pc.

I'd vouch more for old laptops, which are generally not upgradeable, come with built-in UPS, if you remove the screen is as thin as a notebook and can handle low usage. Then you can connect either directly or via other interfaces a bunch of disks and you're golden.