The bug wasn't just corrupting the data in the file after the weird sequence of API calls.
It was corrupting it with data from other files on the disk, which could have been sensitive.
Where this was particularly troubling was that an unprivileged user could then use this as an exploit/attack on the system to get it to leak pages of system files. This is where fun stuff like pass-the-hash begins.
My reading of the bug is that it would write random data from memory owned by the OS, not necessarily other files on disk. Certainly no better (and could very well be worse), but just a clarification.
(Going back and reading the post again, it seems that it's even worse than that, since a virtual environment hitting this bug could get data from the host. This opens an attack vector for a guest to bypass the hypervisor and compromise sensitive host data. I don't know if data from other VMs on the system could also be exposed in this way, but that would also be quite bad.)
Anyway, I think my rambling caused my point to get lost, because I wasn't trying to argue that Microsoft are justified in their "don't do that" comment. But no sane, realistic OS can prevent against every hare-brained thing a driver developer is going to do.
It was corrupting it with data from other files on the disk, which could have been sensitive.
Where this was particularly troubling was that an unprivileged user could then use this as an exploit/attack on the system to get it to leak pages of system files. This is where fun stuff like pass-the-hash begins.