blocking off the oil to the driver's side is just a way to direct the oil pressure back to the main bearings rather than have it leak away around 8 lifters that don't need it.

For high perf street motors I usually block off the driver gallery but then drill a small hole (0.090) or so in the plug so that oil still flows, but at a reduced rate. I just haven't gotten comfortable with the idea of completely blocking off the lifter gallery on a street car.