Earlier quoted context omitted.
I don't agree with what that quote is saying. Americans living abroad can still vote, and therefore are still represented. It's true that Americans living abroad don't all have the same representatives, since they're represented by the elected officials from where they're registered to vote in the US, but that's a far different thing from saying that they have no representation at all.
I'm an american living in Norway. I can vote for president, sure. Supposedly I am allowed to vote in the district I last lived in the states, but trying to get any of this done is difficult, especially since the state I live in requires proof of address for voter registration. I understand the likelihood of the representatives there to care about any issues I have is pretty minimal. After all, americans living abroad…
I personally never had any trouble voting while I was living abroad. It was slightly inconvenient to track down a fellow American to witness it, as required by the absentee ballot I was issued, but no big deal.