I think a $150-$200 premium for a single E5-2670 setup is worth it over a dual 1366 X56xx setup.
Your delta is the whole system cost I had. About 150 for mobo, 2x CPU, and RAM. Even if the power use is 50W higher, that's noise on my power bill. Perhaps not something to be proud of, but it is what it is.
I'm not saying you are wrong, just pointing out that everything is relative. To do a dual E5-2670 system from Natex with similar RAM amounts, was about $500 at the time. I can run my box for a long time on $350 worth of power. By that time, 2670 setups or maybe even Zen based systems will have come down enough to consider for me. And my CPU use is rarely over 20%. The only reason I'd consider getting faster CPUs is that I can see a possible increase coming from Plex transcodes as I set up more players.
Granted, you're talking about single chip setups. There wasn't a lot of single chip around at the time in server boards, and IPMI with KVM was a requirement for that build. While you could use a dual board with a single chip. At the time, that would shave about $70 off the total. It looks to be about $85 now.