The Ad Hominem Fallacy

The strength of a person's argument has no bearing on whether the person gains something from defending it. I can't refute an abortionist argument for abortion by just saying, well, he makes money from abortions. What do you expect him to say? Bernard Nathanson was an abortionist who became pro-life. Lots of other abortionists have done the same. The Protestant Reverend Rob Shank used to work for groups that fought abortion and wanted to make it illegal. He now supports legal abortion. But mostly this argument is a variation of the ad hominem fallacy, one we shouldn't have in a debate.