Catwoman is Batman's most popular love interest, and probably has the larger shipper base within Batman fandom. I think they're an overrated pairing, but I also know I'm the minority in that regard. She is crucial to Batman, but this wouldn't be the first or even second Batman film series that had Catwoman not show up in every installment. In any event, it's very likely that even if she doesn't show up in the second film, she'll be back for the third, which would be more than what the character got in the Burton/Schumacher or Nolan series.
And I doubt people are scared that Catwoman could upstage Batman. Joker is the only Batman character where I could see people actually being worried he'd upstage Batman, and that's because he's done it more than once.