The claim that Earth would not have representation if it did not have a leader of its own sounds ill conceived. If anything, Earth would have better representation than any other world, as its direct representative would be at the very highest level!
Local defense forces do not seem to feature in Star Trek at all. When defenses at Vulcan or Betazed are mentioned, no reason is given for us to think that these would not be Starfleet forces. On the other hand, when such forces are discussed, the infantry element never really enters the picture, as it serves no tactical role in the situation: every local fight ever described in the above fashion hinges on the presence or absence of space forces. Perhaps local ground forces might exist, for those exceptional situations where ground forces are not irrelevant? It's odd, though, that they are never mentioned let alone shown, neither in a "home guard" role (which would have been prominent in "Paradise Lost") nor even in a police role.