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.
