I'm in general agreement, but your point about a reach is questionable. What is a reach? Nobody ever knows where NFL teams generally grade each player. I am certain that even among NFL teams that there are wide disparities in the grade of many players - that is, team A may grade Joe Smith very highly and Team B may grade him considerably lower. It is an oversimplification to think that there is just this list that ranks players in order from 1 to 200 and everyone agrees with it.
In addition, let's take the QB example that you used. Let's say that our beloved Bills have QB Billy Bob Bomber rated as the 40th best player in the draft. If the Bills are picking 20th and they desperately need a QB (most important position in NFL), I might argue that they should go ahead and select Mr. Bomber. If they wait until the 2nd, even if they trade up into the top of the 2nd, that guy might not be there. The argument could be made in the reverse too, so there is no clear cut answer.