My main issue with this type of question is that people will take this (or other popular questions) as the catch-all type of problem to ask in any interview for

My main issue with this type of question is that people will take this (or other popular questions) as the catch-all type of problem to ask in any interview for any candidate. If you are hiring a generalist or you are in a company that writes code in everything under the sun (Experience in Java, Ruby, Python, iOS, and Javascript preferred!), these questions are probably appropriate. But when you are hiring a web developer to work on your Rails app - the interview question should not revolve around optimizing space complexity or a recursive algorithm that exploits symmetry. The question should be more targeted to what the day in and out work will be - write some RSpec tests, fill in some controller methods, create a simple model. Use that as the filter. I don't think anyone applying to a Rails gig is going to balk at doing that kind of interview and I think it will be a better predictor than if they can find the largest substring palidrome in O(n).