Google: 90% of our engineers use the software you wrote (Homebrew), but...
At a certain point, your resume should speak for itself. The fact that experienced engineers with impressive resumes are put through these types of interviews is insulting and frustrating to the interviewees.Succeeding at these whiteboard questions requires weeks of preparation. You need to practice, practice, practice.  After enough practice, you are pretty likely to pass. So ultimately, it is more of a test of "how much do you want to work here." If you care enough, you can pass.  That said, these types of interviews do favor fresh grads over experienced programmers (you have algorithms and data structures fresh in your mind), which means that they are flawed in IMHO.