There is still significant sharing that can be achieved inside a VM, plus, a lot of the sharing come from zero pages (full of 0) which is still performed accross VMs.
Another benefit of the salting mechanism is that it allows the administrator to define groups of VMs that are trusted in which sharing will be performed.
disclaimer: I work at VMware and wrote the salting code.
I would guess if you're a big VM hosting provider and you have thousands of VMs all running the same version of Windows or Linux distro, that it could add up to some real savings to have them share common pages.
Conceptually, it's safe. UNIX distributions routinely do the equivalent operation within single machines, it's a fundamental part of their operating model.
It's just that in the face of defective hardware, it's not safe. But this is not surprising, because nothing is safe, so it isn't particularly a criticism of page sharing. This specific attack may have used it, but Rowhammer is a powerful tool. This is not the only way it can be used; it is merely an exemplar.
