In club cricket opening is the only place to bat, as any fule no. If I'm not opening, I'm not playing, pal. It offers the opportunity [not often taken, admittedly, but glorious when it is] to do what Virender Sehwag did in the first match of this World Cup - play a swaggering, gratifying, alpha-male innings whilst developing some sort of injury that gets you out of fielding later on.
Club cricket is for the most part a short game, hence the imperative not to waste any of it. In the pro ranks there's always another innings, but the shape of 50 over cricket gives opening an appeal that other formats don't for the alpha batsman. In Test cricket it is the ultimate examination of skill and nerve, and thus suited only to certain players with particular temperaments. In T20 matches, someone always gets out early, so the number three is essentially an opener too. In 50 over cricket, though, its appeal glistens in a particular way: the powerplays, the high-vis new ball that cracks off the bat, the bowler concerned with defence and attack, the chance to build and pace an innings.
It's no surprise then that many of the biggest, most anticipated and feared batters of the modern era open in ODIs: Hayden, Gayle, Gilchrist, Sehwag, Tendulkar, McCullum, Watson, Jayasuriya [the proto-ODI opener] and so on. No surprise either that KP fancied adding his name to the list.
It's a smart move in lots of ways. It suits his game on the sub-continent, it offers him a challenge, it boosts his misunderstood psyche. Pietersen is indisputably a big-game player, recently to the exclusion of almost everything else. As the T20 world cup proved, he's well suited to tournament cricket, with its unfolding narrative and building momentum. He's somehow attuned to things like that. For a deeply idiosyncratic man, the most idiosyncratic place in the order might be perfect.