Mike Jenkins has been an extremely inconsistent player at a position that demands consistency. The Cowboys jumped all over Morris Claiborne for a reason after they replaced Newman with Brandon Carr, imso.
Here are the last three years of YPA for Mike Jenkins according to K.C. Joyner:
2009: 5.4
2010: 11.4
2011: 7.1 (12 games)
During the past three seasons, Jenkins has averaged a very poor 8.0 YPA. That number goes up to 9.25 YPA for 2010 and 2011. That is really, really bad. Consider that Newman was at 9.7 last year. Furthermore, Newman consistently rated among the very best corners in YPA (from 2004-2007). That doesn't include Newman's stellar 2003 rookie year, either. In fact, Newman rated number 2, from 2004-2007, according to a Rafael Vela analyis, via Joyner's stats. In 2008 and 2009, Newman was still a top-notch CB, when healthy, and he made the pro bowl in 2009.
While Jenkins was one of the very best CBs in 2009 and deserved his pro bowl status, he was one of the very worst CBs in the NFL during 2010. In 2010, Jenkins' ability to defend the deep pass and intercept the football dropped off the table. For all of the hype about Jenkins' ball skills, Jenkins has only averaged 2 INT per year, and he has three seasons with just 1.
I'm not sold on Jenkins having big time trade value unless a team just gets desperate and carelessly throws Dallas a high valued pick. I guess you could always try. We'll see.
Also, I'm not sold on this big "competition" between Claiborne and Jenkins. I think the 6th pick of the draft starts day one, and I think that choice has already been made. Newman was the 5th pick of the draft and won the Thorpe Award, just like Claiborne. Newman was a day one starter and had a great rookie year. Claiborne should be able to do the same thing.
I think Jenkins is a starting caliber CB, but I'm not sure just how good he really is, because he hasn't demonstrated consistency. In fact, Jenkins' play has been all over the place.