Well, right off the bat, I have to wonder why the hell us North American viewers have that stupid Amy's opening narration for this episode. Amy and Rory have only one scence cameo in this episode so to have Amy telling us of meeting the Doctor as a child and joining him on adventures as a grown up seems really pointless in this episode. I accept Karen Gillan and Arthur Darvill are still in the credits due to contractual reasons, but there was no need for the opening narration. Actually, there's never any need for it, but that's besides the point.
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Because, like it or not, it's part of the opening credits for North America right now. It also get's people who've never watched the show up to speed on the concept.....like the whole "Space, the final frontier" thing in Star Trek.
Frankly, I've come to like the narration as it adds something fresh to the opening credits without changing the main credit sequence. It'll be interesting to see what happens to it when the companions eventually change.
It was left out of the North American DVDs for the first half of the season, so it's not really "part of the opening credits." Besides, Doctor Who does not need an opening narration to explain what the hell it's about. It lasted 47 years without one. And Star Trek eventually dropped the "Space, the final frontier" intro anyway.