This, to me, underlines the similar-ness between the pagan and the christian as modes of control. They both are frameworks around which the stability of a society is built (at least then, for in my opinion, religion has been replaced with democratic law--at least in the west). The old and the new work together to take out the chaos of the sacral king's sin, to bring about the end of the beginning of the new society, of which Arthur's Camelot and Britain were the progenitors.
This is also underlined in the scenes where Percival seeks the Grail. The Grail is turned from the cup of Christ back into the celtic cauldron of plenty, while keeping the overt chirstian overtones in its iconography.
It's kind of a disappointment that these weren't explored more on screen, but these definatley were not in the filmmaker's balliwick when they were making the film. They were just trying to create, in director Boorman's words, "mythic truth", which to a degree they did. Perhaps they were smart to leave thesubtext the subtext, and just tell the story.