Seems like a good idea in principle. I would be in favour of notifying the user somehow (email? no point in an inbox notification if they're not using DW) when they become "inactive", on the principle of not giving people captchas when they have asked not to have them without explaining why. But, the notification might be perceived as spammy...

If we already have the last login date easily accessible in the database, would this be an acceptable way to determine whether somebody is active? I have little idea what I'm talking about here, but I imagine that it would be easier to implement (but a counter such as you describe might also be easy. I'll leave that to the people who have a clue ;-))