While at ISU in the early 2000's, I had the opportunity to go with a group of about 30-40 students to the state police academy in Des Moines as volunteers. The trainees were practicing the field sobriety tests on us, and we were given various controlled amounts of alcohol so that we were all at different levels of intoxication. I was on the low end of the range - just a couple of beers and well below .08, and I had no issues with any of the tests. Overall, it was pretty obvious that the higher a person's BAC, the worse they did on the tests in general. Of course, there are exceptions, but based on that experience, I don't agree that they're completely useless in determining the ability of someone to drive safely.
*** Edited to add that I only wanted to share my anecdote in regards to the discussion around field sobriety tests and I am in no way defending the cop in the video! ***