Sorry, I got lost a little in the second part. I assume you mean "a negative influence". How exactly discussing viewpoint, which is not believed by the person, influences the real-world decision in a negative manner, considering that more information is processed, not less?
Lets say two presidential candidates are having a debate. Does it matter if they actually believe in what they are arguing? Of course it does. Or two prospective parents discussing how to best raise a child? Of course it matters if they actually believe what they say.
Presidential "debates" to me appears as a PR campaign. I guess there indeed it is important to know exactly what candidates are standing for and proving their view is the right one (i.e. aligned with viewer's worldview)
With parenting - I see no issues. And I do use such an approach, but my spouse, I believe, trusts me.
So... It looks like the problem is - when the debate is used to judge world view of the person to decide if one would like to trust them - such "devil advocate" arguments are harmful. I believe it is just one possible use of the discussion, but I will keep in mind that and try to avoid arguing from some other perspective if I see the person is trying to figure out if they want to trust me or not.
