If I remember correctly Oro developed the cursed seal as a way to absorb and control nature chakra, which he meant to be implemented the way Juugo does it, as a means of using that skill himself when he swapped bodies. He did not nes. give people the cursed seal as a means of "creating an army" but he gave people who he saw as potential host the mark as a means of testing their tolerance with it (an most of which were failed experiments in his book). Sasuke's body type and training suited it best and as such that was his successful candidate. So yes, sasuke already has a leg up in terms of natural chakra.... just not from a particular beast, i.e. snake, frog etc. Whatever the hell Juugo is the key to sasuke's natural chakra ability, at this point in time.