50 Book Challenge #38: Harry Potter and the Half-Blood Prince

After the bar, the first thing I did to purge my brain was to wheedle Will out of his copy of HP6 (which he had thoughtfully hidden so it would not tempt me) and read it in one gulp. It was far superior to the fifth volume, mostly because Harry was not an irrational, whiny prat. My only complaints were that the revelation of the Prince's identity was a bit clumsy (was it really in character for the person in question to refer to himself as the Half-Blood Prince?) and that the Peter Parker imitation at the end was one more tiresome iteration of a paternalistic impulse by a hero with reference to the autonomy of his/her loved ones.

One real, substantive magical query: why does Voldemort not require that Death Eaters swear an Unbreakable Vow to obey him? Does it have something to do with some vulnerability he might be under with regard to the Bonder? Does he obtain sufficient benefits from the perception of strength potentially gained from not requiring the Vow such that the efficiencies reaped from not having to pursue and kill traitors to the cause are outweighed?