Interesting question. I don't believe someone should be able to vote just for being a resident. I will get my citizenship within the next couple of years, and even then I'll feel a bit strange about voting here I think. It only takes 3 years to be able to apply for citizenship, if one wants to vote, it's a short wait.
As for people voting absentee, I don't think it's fair to say that none of them would understand the issues or candidates or whatever. I'm sure there are many resident citizens who don't understand the issues and cast a vote. In this day and age, it is not difficult to research the candidates from abroad and cast an intelligent vote. Since when is first hand knowledge required for a citizen to vote? It's not required to be a candidate, lol. I wouldn't bother in my home county/city's local elections, but I would for state and federal elections. (That said, I did not take part in the last election because I didn't get my registration done.) If you make enough money here, you will be paying federal taxes (and you'll be entitled to collect social security on retirement I believe). Do you want to tell the ones who are paying taxes they have no right to vote anymore? Are you going to say that only those paying taxes can vote? Only the higher income absentees will be voting then, hardly fair.