If you're looking for a senior developer with 5-10 years experience in a language it's a useful gauge of experience and whether they think about the language that they work in , particularly if it's something that might bite you. eg. What does the following do in Python 2:

  try:
      my_dict['foo'].append(result[1])
  except IndexError, KeyError:
      my_dict['foo'] = [result[0]]

And you ask interviewers language questions to find out the same thing - namely, whether they really know what they're doing. I'd rather do interesting work, and not get stuck explaining list comprehensions to a bunch of people.