Philosopher as Ruler

Socrates tells Glaucon that the last crucial component for of the city of necessity to be possible is for philosophy and political power to meet, and thus the rulers must themselves be philosophers (473d). He hesitated to mention it because he sees it as a paradoxical notion. Why does Socrates think that the idea of a philosopher with political power is a paradox?

Mr. Bloom provides a lengthy and (I think) helpful note on this subject. He says that "the philosopher has neither the desire to be a ruler nor would he do what is necessary to impose his rule on unwilling people" (Book V, note 36). It seems as if this description is confined to one type of philosopher, and is likely the sort of philosopher that Socrates and Glaucon agree on ("...rather than lovers of opinion those who delight in each thing that is itself" [480a]).

Maybe Socrates and Glaucon do not think that the philosopher will do what is necessary to impose his rule on unwilling people because the philosopher does not know the form of the good and maybe cannot know it. And without such knowledge the philosopher would have no preeminence for ruling. Hence Socrates' hesitation with completing the argument, as he claims that he is merely "in doubt and seeking", and does not claim to have discovered or established anything about how to actually make a city (or a man for that matter) just (450d).