serial port pin headers are not he same thing as having a serial console.
how many of these Intel server motherboards are single socket?
I've purchased single-package systems from HP, IBM, Sun and other tier-1 vendors. In every case the system that I received had a multi-package motherboard with only one package installed in it.
In that situation, the most cost-effective way to get compute capability is to go ahead and fully populate the motherboard. Even if that (usually second) package isn't as efficient as it would have been in a separate system (with no contention for RAM bandwidth, etc) it's still the best thing to do because the marginal cost of adding it to an existing system is so small.