Wonder if there's a selection bias here; VirtualBox may be less stable than native hardware or commercial VMs, but it's user base is also using VirtualBox for different scenarios than native users.
I know I use VirtualBox all the time to try out "risky" behaviors that often end in a crash, because I know I can just roll back if it doesn't work out.
The issue is that virtualbox is crashing the host kernel, not the guest machines. And specifically that it's polluting Red Hat's automated bug reports from those crashes, and thus wasting QA effort. Being able to filter those out by the taint mechanism in the patch is important and useful information. Read the thread, it's a technical and process discussion, not a flame.
I know I use VirtualBox all the time to try out "risky" behaviors that often end in a crash, because I know I can just roll back if it doesn't work out.