If I could regularly utilise all 12 threads I would buy an i7-3920K as when you look at the overall cost of a PC including the screen, software etc the difference in price between a system with an i7-2600K and an i7-3920K seems reasonable considering the performance difference. Roughly an extra $240 for the CPU and $120 for the mobo so ~$360 in total. So for even a $1,000 system this equates to an extra third on the cost for up to an extra 50% in performance. If the percentage gain for performance is higher than the percentage gain in price I consider that a good deal.
While i don't disagree with your statement, there other things to consider, like Ivy Bridge is like 4 months away, with rumors saying it will be 10-20% faster than Sandy Bridge, that should put the i7 3700K right next to a i7 3920K, for $300 less. Personally i would take the ivy bridge more because there is not that many apps that will take the advantage of the extra cores, so i would prefer 4 cores with similar cpu power as 6 cores, specially in games i dont know even 1 that uses more than 4. Now there might be apps specially design for multithreaded and takes the advantage of 6 cores 12 threads... but to justify the $300 extra... and to justify a much more expensive mobo, not to say power consumption.... its just not worth it for the average user.
Once Intel change their release of their two platforms the lower end becomes much more attractive. What i mean is that when the x58/1366 i7 920 was release was a big jump over 775, there were no 1156 on the market to compete, like a year later we saw the 1156 which offer good value, performance and consumption. but the i7 920 still hold its own, Intel shoulda released Sandy Bridge E X79/LGA2011 to replace the X58/1366, but instead they released Xtreme CPU with 6cores for the X58/1366 and they went main stream with Sandy Bridge 1155 and we got really cheap cpus that could compete with $1k i7 980x, thus the need of new platform, but with them changing their order releases, with mainstream before high end, they are imo just killing their high end market, i would have gladly payed for Sandy Bridge E i7 3930K $600 at the begging of the year (and intel delaying the mainstream 1155 for November, like right now, and ivy bridge none E for Nov 2012), as i would have gotten a cpu that would perform as Ivy Bridge two years in advance for the extra $300, but now its backward, with Sandy Bridge E you get CPU that cost almost twice as much that in less than six months time there will be a similar with Ivy Bridge, with much less cost on the other hardware needed to run.
Intel needs to revert their releases for their E line, being before the none E, to make sense into spending that much. Either way for me its Sandy Bridge for now, probably CPU upgrade on Ivy Bridge and waiting for Haswell (not E), ill skip the full E line at least for a couple of years.