Story-telling wise, 3PO getting blasted is so they don't have to figure out "but what is 3PO going to say about this without ruining the dramatic tension?" Notice how 3PO isn't around or is shut down for nearly every dramatic scene in the OT. Obi-Wan tells Luke about his father, 3PO is shut down. Han and Leia discuss where to hide, 3PO is shut down. The gang is captured by Vader, 3PO is in pieces. Think of his dialog during the carbonite scene, its expository and kind of goofy. It would be really hard to have him being quiet unless he was shut down/not present.