As important as arguments are, it's just as important (or even more-so) to see where the arguments are coming from. If someone was brought up to believe a particular holy book as being true, it's relevant to point that out. After all, other people are brought up to believe a different holy book.
Of course that doesn't reflect on the truth of the claims, it might be that one holy book really is true while another is false. So to take that fact of motivation and contingency to extrapolate a falsity is being unreasonable.
Deflecting an argument on global warming for example by attacking someone's source of information would be another example of this, it's attacking not the argument itself but the "argument behind the argument". In come cases, this meta-defeater might be justified - someone parroting statistics from an unreliable news source is such a case. But I think there is a danger in overusing this approach.
As humans we have many unconscious motivations for things, and it could very well be that our unconscious motivations are the ultimate reasons behind our reasoning. Hence if one could have a theory of unconscious motivation that explained argument X, then how different is that to someone parroting an argument they read in a newspaper?
The difference is two-fold, firstly it's a matter of known attribution. A creationist quote-mine for example can usually be traced back to a creationist website. How do we trace back unconscious motivation through the brain? The second is how easy it is to create a story of unconscious motivation. Any number of stories could explain motivation, and one that might be true for one person might not be true for another.
And finally I think the greatest danger is that if we're going into the realm of the unconscious, then any hope for rational discourse is lost. We don't have access to our unconscious biases, so how can we reason at all? It sets the dangerous precedent of not addressing anyone's argument and instead trying to weave an acceptable meta-argument that can explain the argument itself. Once we start digging into the reasons for the reason, then we're losing any capacity to engage in rational dialogue.
This is not to say that people aren't biased or that people don't have unconscious motivations, or that we shouldn't strive to find our own. But for a rational and reasonable dialogue to work, unconscious motivations should be at best a framework rather than a centrality of discussion. We all bring our individuality into every discussion we enter, and that's to be expected because we're human. Dealing in unconscious motivations, however, brings us into the territory of unfalsifiable accusations - after all, who can adequately deal with what is said to be their subconscious motivation?