[absitively]

1) If you're trying to hire a Ruby developer to write Ruby applications and that's not what you're screening in interviews, you're not proving anything relevant to the candidate's ability to do the real world job. Between not having anyone to interview and an algorithm interview, I get where you're coming from. But if there is any other choice, you shouldn't be giving the technical interview to someone whose practical skills you can't assess.

2) There are a billion things to learn as a technology professional. The time you spend practicing specifically to be able to whiteboard algorithms for interviews is time you're not spending doing something else. Something that will be more beneficial to you and to your business on a day to day basis than learning how to interview well. So choosing to do those other things instead doesn't show that you're not engaged.

[avasylev]

Yes if you need specificly Ruby dev, you ask Ruby (though only if you need it now right away, may as well hire contractor for a few months in this case). Imho smart developer can pick up Ruby fast (or other stack fast) and that's the benefit of algorithm related question - you don't have to pass on candidate that don't have experience in your stack.