I mostly like this answer. It depends on the individual's time and how they balance against the needs of the rest of their life. I can judge a certain amount of motivation from outside contributions, but I understand it is not the only angle to learn about that.
I would add that you should hold an intelligent dialogue with your interviewee. This means doing more than just asking a question. Follow up. Not only find out if they have done something, but figure out if they understood what they were working on. Find some specific details in it. If you need a nice canned starter question for this: "What was the hardest problem you ran into when implementing X?"
In other words, interviewers should have filter dialogues, not filter questions. Genuinely learn about your interviewees. You must direct the conversation intelligently. That is how you can find the diamonds amongst the poor salesmen.
