A longer rod, Will allow you more reach and aid in being able to keep more line off the water...and you in better contact with your fly in adversely differing current flows..dern, that sounds to techie to have come from me....let me try this again...hummm...

Can I just recommend that you try and find a copy of Joe Humphreys Video/DVD A Casting Approach to nymphing Tactics. or something like that.....It'll pretty much also apply in many ways to getting desired dead drifts to a dry in certain situations as well..Great section covering 10' rods and the advantages of them and deferring lines...