As someone who has given hundreds (maybe thousands) and received dozens of these types of interviews, it's really not.
Generally speaking I'm looking for at least 2-3 of these abilities, depending on which question I'm asking:
1) Can you reason about data structures and algorithms when looking at a problem you haven't seen before? 2) Can you communicate your ideas effectively? 3) Can you integrate new information from a colleague while problem solving? 4) Can you write code that makes sense? Do you understand basic programming concepts? 5) Can you read your own code and reason about it?
There is at least two hidden attributes that I'm not testing for but do affect performance, so I try to account for them:
1) Are you comfortable with me, a relative stranger? 2) Can you do these things while dealing with a high pressure situation?
You will encounter high pressure situations at work but often interviews feel more high pressure (to some people) than most daily conversations about engineering.
That's it. If I can tell a candidate already knows "the right answer" to the problem, I'm usually disappointed because I'm more interested in watching them think.