Why is it that the presidential candidate who so consistently says the smartest things (most recently here but most especially here) also keeps saying some of the dumbest things?
Maybe the cost of a candidate who speaks impressively and intelligently is someone who also has occasional verbal diarrhea. It's a cost I'd personally gladly pay to get a president who's actually really really qualified for a change, but there's almost no chance such a person could get through the modern media gauntlet.
As I think we demonstrated pretty well in All the President's Spin, the best media strategy for a modern president (or president wanna-be) is to tightly control everything he or she says. It's not very interesting for those of us who pay attention, but most of us don't pay that much attention. And we all know what kind of speech - impressive policy discussion vs. verbal gaffe - is going to generate the headlines and the late-night jokes from which those who don't pay much attention get their political information.