As I also posted on David Brin's site, I don't think it makes sense for Voldemort to let Harry have the wand just because he believed Harry was unable to use it. Precautions don't work that way--taking precautions means that you have to try to stop some things even if you don't believe they'll happen. Just saying "I don't think he can do X" is insufficient reason to avoid taking precautions against it. Proper precautions depends on the size of the mistake you'd have to make in order for X to be a danger. And "I don't think he can use this weapon" has a lot smaller mistake connecting it to danger than "I don't think he can use this non-weapon as a weapon".