Originally Posted by Scrap Irony
Disagree. You value a prospect not by what he could do, but what he could/will/might do when compared to others at his position.
When comparing Alonso's ceiling, floor, and likely production with with the 1B already in the major leagues, in the high minors who're likely to be in the league with him, or the "value" of said prospects below him, Alonso should be negged a bit for Votto's performance. It affects his "value" to the Reds (as a trade piece and as a major leaguer) and as a 1B overall.
I think you're confusing trade value and prospect rankings. Yes, first basemen, even good ones, lack the type of trade value they would have if they had the same offensive numbers at another position. But when you do prospect rankings you're rating that player's ability to excel at his position in the majors, not his trade value.