I have this rule that when I hear someone make an argument and they give me two or more reasons they are right, I always assume they are wrong. Some examples:
We went to Iraq to prevent WMD terror . . . and to spread Democracy.
Cash for clunkers will stimulate the economy . . . and lower carbon emissions.
Affirmative action is helps level the playing field . . . and diversity is good for every college student's education.
We should make Medicare a voucher system to increase competition . . . and to reduct the deficit.
Whenever you hear more than one reason to support a policy, odds are someone is just trying to find something that sticks. Why would people do that, why not just offer the one compelling reason that convinced them? Probably because they know you disagree with them on that point, so they hope they can sway you with other reasons -- reasons that they don't even find convincing.
And that is the generous interpretation. Sometimes people give you a lot of (bad) reasons because they want to hide the real reason they favor a policy. M.D.s will tell you that "brain drain" of medical professionals causes shortages overseas, hurting the poorest, and lowers quality standards in the U.S. . . . but not that it increase supply putting downward pressure on wages. The foreign aid lobby will tell you that the U.S. spends lots of money screening immigrants for TB each year and that dangerous strains of TB from overseas will eventually kill Americans . . . but only when they can't sell spending on overseas TB treatment for humanitarian reasons.
So whenever I hear two reasons to support a policy I assume that one of them is bullshit and that the other is so weak not even the person saying it believes it.