Setting aside the pressing question of measurement and bias, it's not at all clear to me that it is just to apportion power according to ability and talent.

Consider Rawl's original position argument: https://en.wikipedia.org/wiki/Original_position -- sure, if you perceive yourself to be (or actually are) of high merit, a meritocracy sounds great. But defining a just society requires one to consider the possibility that one could be in the position of being incapable of meeting the demands that a meritocracy requires of its participants in order to flourish.