It's true that *non-persistent* linked clones lose state between reboots, but persistent ones do not. There are use cases for linked clones that extend beyond space savings, such as the ability to refresh a bunch of VMs from a source image while retaining user data (maybe there's a way to do that with standard clones that doesn't hose user content, but I'm not familiar with it). The dedupe ratio is impressive if borne out by experience, but I imagine that once users start pumping their own preferences into those cloned VMs, the dedupe ratio will drop significantly.
Honestly, I'm always a little dubious about relying on deduplication and compression for space savings. All it takes is a few corner cases with data that's hard to compress or dedupe, and suddenly you're scrambling for space or performance. Admittedly, VDI is the low-hanging fruit in this regard, with a generally large amount of static data relative to overall capacity, so perhaps GreenBytes can carve out a niche for themselves in that space.