I suspect it's more common than anybody would think. A disturbingly large number of stories I've heard about regarding "consultants" (from numerous points of view, including consultants themselves) seems to indicate there's a huge quality variance in the field.
How common is this?
I had a similar situation. I didn't challenge the developer directly, but when asked to justify why our project was falling behind I sent a summary of the VCS log to my manager. The developer hadn't committed anything in five years! Management promised to sort things out, but it's over a year and she's still employed.
Throwaway account, because this is one of the reasons I won't be employed there much longer...