First, while there may be problems with mail-in and other absentee votes (such as concerns it isn't actually the person listed who is voting), mail in voting is in many ways superior to going to polls. In fact, some states vote ENTIRELY by mail (Oregon is one of them), and those states consistently show more voter turnout than states which require people to go to polls.
Second, we should be encouraging people to vote, regardless of mail in or showing up at the polls.
Third, I'm not sure why anyone cares how anyone else votes. There seems to be this overwhelming desire by some people to tell others how to live their lives, in everything from how to spend or use money to how to vote. It's America, it's a "free country" - let people make their own decisions. As long as they aren't breaking the law, you shouldn't care how they live their lives unless you are somehow otherwise negatively affected, and even that can be debatable dependent upon the circumstances.