By the time of residency interview, step 1 is an old achievement for most applicants. Timewise, it's the same as using your PSAT score for justification for admission to college. Sweet, you were an awesome 10th grader, but how do we know your ability now? Where's that recent SAT score? Have you started smoking pot and skipping class like most of your fellow seniors? The step 1 gives residencies a picture of what you were doing 2 years ago. OF COURSE they are going to be interested in knowing how you've been doing recently. Programs are scared of getting another <insert bad resident name here>, which every program has had and still talks about who had a killer step 1 but had no social skills and a bedside manner that embarrassed the department.
Also, everyone is blowing step 1 out of the water these days. 240 used to be an astronomical score 20 years ago. Now it's the average at some schools. There's not really much difference between an applicant with a 245 and a 255. That comes down to a handful of questions right or wrong. I seriously doubt that programs are going to take the 255 guy over the 245 guy because those extra points are going to make him a better resident. Past a certain score, other stuff is going to be the deciding criteria.
Watch step 2 become the new step 1 over the next decade or so. Anybody with half a brain can figure out that showing up with a 245 step 1, a pass on CS, and a 260 step 2 is far better than showing up with a 255 step 1. The first guy has everything in the bag and has perfectly wrapped his medical school career up into a neat little package to be presented to the programs, whereas the second guy has some explaining to do about what he's been doing for the nearly 2 YEARS since he took step 1.