ACI 336.3R-93 3.6 also describes Piers socketed in rock as "...The drilled pier consists of a heavy wall permanent casing fitted with a cutting shoe and seated into the top of rock." I believe there is an ACI 336.3R-14 out but I don't have a copy to see if they've changed this description.
I would expect a temporary casing may be required because of the transition from earth to rock. But I visited a number of drilling and DOT web sites and none of them indicate that permanent is necessary or required.
IBC 1810.3.9.6 indicates: "Socketed drilled shafts shall have a permanent pipe or tube casing that extends down to bedrock and an uncased socket drilled into bedrock, both filled with concrete." Any ideas why IBC requires permanent casing vs. temporary casing?