They don't have to. Why is it so hard to do two things at once - in this case, evaluate both motive & argument?
Doug
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You can, but people don't usually talk about motives when they think the argument has merit. Then the motive is seen as nothing other than expressing a rational point of view. It's usually when arguments are deemed spurious that a fair-minded person goes for the motive ("Why is this guy making such a self-evidently stupid argument? There must be something else to it.")In other words, motives are normally invoked when hopes of responding rationally have been abandoned, so to evaluate both the argument and the motive mostly means you have disimissed the argument. It may be perfectly OK to do this, depending on the argument, but this is what you're doing.