That's the question, isn't it? But he's not .280 / 20 / 80 RBI guy "when things he's going well", like BH34 said.
This board had the same debate about Zim not being a 3 hitter in 2008. Zim answered with his 2009 and 2010 seasons, which is what he can do "when he's going well".
I'd of course agree that some years he would be better suited for another spot in the lineup as Harper grows and some of the supporting cast steps up.
Also, he's 27. So hopefully he has a few more 2009/2010s in him.