:) Obvious or not, one does still occasionally hear "give us more cores instead".
I agree about the communication cost. Higher clocks are harder - quadratic increase in power. For shared-nothing problems, an array of wimpy (low frequency) cores is pretty good. It seems to me that wider, lower-frequency cores are a good compromise: beefy enough (thanks to vectors) to reduce the number of cores required, while still power-efficient due to both vectors and lower clocks.
That should be a pretty obvious conclusion--higher-clocked cores is better than wider cores, and wider cores is better than more cores.
Communication cost between concurrency domains for superscalar/supervector: min <1ns
Communication cost between concurrency domains for multiprocessors: min 10-100ns