Also, even though they could run their load on a fraction of the hardware it would probably not make a lot of sense to optimize for that, as the potential cost savings are relatively small (servers are cheap).
First off, 750 hosts don't have 15k CPUs. The ganglia metric likely refers to cores and doesn't account for hyperthreading.
And, again, 750 hosts is not too bad for an operation the size of Wikipedia.
A running server doesn't cost much in the grand scheme of things. You can infer that from the fact that ISPs will rent you one for under $30/mo and still make a profit on you.
Consequently the potential savings may not be trivial, but they're hardly big enough to justify tying up large parts of the team in an effort to squeeze out the last x%.
