Why pull up on the rollout after landing, after the nose wheel is down? I'm a student pilot learning on a Piper Cherokee 180.  When I land and the nose wheel touches down my instructor wants me to keep pulling back on the stick through the landing roll.  

For soft field landings this makes sense as you don't want the nose wheel to dig into the ground and tumble over.  However we are landing at an airport with hard surface runways which routinely experiences gusty wind conditions.  It seems to me that if I keep pulling up and a gust of headwind hit us shortly after the nose wheel touches down that the gust would lift the nose wheel back off the ground.  This of course would not be a good thing.  So in my mind I think it would be better to ease off the back pressure once the nose wheel touches down.  What are your thoughts?

Note:  My instructor was a crop duster so I think he just carried over the soft field landing technique and is just teaching this because this is what he learned.
It is a soft field technique but is generally applied on hard surfaces for the sake of the aircraft. There are a few main reasons; 


Take stress of the nose wheel and nose gear assembly which is a bit more fragile than the main gear assembly. This is particularly true in aircraft equipped with a steerable nose wheel like the Cherokee (or in retracts).   
Provide adequate clearance for the propellor. If you look really closely the Cherokee has a low enough clearance such that, excess low pressure on the nose wheel and a sufficiently hard landing can cause a prop strike. This is one of the reasons its so important to check tire pressure in your preflight.  
It also helps to reduce the risk of the fated nose wheel bounce which can collapse the assembly.
Generally it also helps to prevent you from simply releasing the controls after the mains touch down or potentially even pushing forward on the column which would drive the nose down and potentially cause damage. 




During training this also helps to build habits that lead to a good landing attitude and speed bleed off especially if you start out landing flat.