Also the key question is not how long they've thought about the issue but whether they have thought about it from perspectives other then their own and how many different perspectives they've thought about it from.

I disagree that that is the key question. I have a model of a person slightly less smart than the average Less Wronger but who has thought quite a bit more about tax policy. These people tend to converge on similar views about tax policy. The fact that this group generally doesn't benefit from most tax deductions certainly may make it easier to oppose them: but that isn't an argument that they are biased. Obviously it is those who benefit from particular deductions whose judgments we ought to be suspicious of.

That's why, after all, these deductions remain in place despite being really bad policy. They benefit the groups that sway elections.