Exactly. I have a thin Nexus 4, but I have to carry a chunky extra USB battery because a) no replaceable battery and b) thin built in battery doesn't last long enough (yes, this is also Android's fault but that's not my point)

It's not only about being thin, but making it light as well. Making it thicker and same weight and/or heavier is noticeable to more potential customers than even more CPU performance.