Imagine you have two applicants or roughly equal aptitude/suitability. When asked why they're looking for a job, both state that they are not being challenged in their current position.
One seems to have done a lot of posting on SO, HN, and so forth during business hours. You like their posts but not that they were made during business hours.
They other has no record of HN or SO posts. Do you assume that the other person was hard at work at the last job and hire them?
You might be right, but then again, it could be that they were watching the clock in ways you can't detect so easily. Maybe they read books about programming. Maybe they read all these sites but didn't contribute. Maybe they took a lot of smoke breaks.
You know that one of the two did a lot of non-job stuff while being underemployed, but you also know that you like their posts. I think it's reasonable to hire them anyways, and also reasonable to pass on them.
But what isn't reasonable, IMO, is to give the candiate with less information a "free pass" by hiring them without further investigation. I would ask:
"You say you weren't challenged in the last job. You must have had some free time. What did you do with it?"
If they tell you about all the extra value-add they created for their employer, great. If they tell you about surfing job boards or reading books about programming, great. If they stare blankly at you, dig deeper.
This is speculative, of course. YMMV.
But you have a point. We all have small spaces in the course of a day where we're not actively engaged in real work (as I am right now). Once can spend it on Facebook, or one can be looking for things to keep the knives sharp.