Pena will start to steal some playing time from Kearns if this trend continues. However, the reason the Reds are reluctant to do so is because Pena has power - nobody is arguing that - but he still Ks too much and doesn't hit for average at all. Kearns has potentially less power but still a much better overall hitter.
It's going to be interesting to see how long the Reds stick with Kearns as the everyday RFer. I really don't know how much longer they can leave Pena on the bench...but for the Reds sake, I really think they need to let Kearns play to get out of his slump. Overall, he's the better player and prospect. Pena is solid too, and maybe he'll get his average up with more playing time, it's hard to say.