Because players in Madden can be changed to play any position. This is another great thing about Player traits, it will potentially add even more realism to position changes. There has been some discussion about being able to change Punters into Offensive lineman and them performing adequately even with low blocking skill ratings. This should not be an issue in Madden 12 because even if position skill ratings work the same as in Madden 11, these player traits should ensure that players can only succeed at positions where they have solid traits.
I love the potential these brings for not only deterring unskilled position changes and signings but for the added depth of signing and drafting players to play better suited positions. For example, a 50OVR rookie QB with good hands, speed, route running and good receiving traits, would have a better chance of being a decent receiver with the addition of Player traits than last year.