He's not needed to score big hundreds regularly– they've got Younis Khan, Yousuf, Inzamam and two openers (or maybe Faisal Iqbal or Salim Elahi) for that. He's in the team more as a leg-spinner in ODI's and a mere striker of the ball, and as a striker, he's just right for his team. In fact, that he's got a few centuries is a massive bonus.
Everyone is looking at him as an individual, more so as a batsman, so you don't get the complete picture. He may not bat with a brain to some, he may not have Tendulkar-esque figures, but he serves a purpose in the team, and quite well. The strike bowlers need support, and he turns in seven overs a match without giving away much and even takes more than a few wickets. With a flat pitch or a massive score behind him, he'll come in and strike the ball very hard a few times and while it may not last long, it may win matches for the team and that's what counts. In fact, he's one of the team's genuine match-winners, which makes him more important than a few blokes hopefully averaging over 40.