Anyone experiencing the raw power of two 5970's is not likely going to be happy with the performance it has compared to cards 3-4 years into the future.
Well lets think for a second. Right now a Crossfire x 5970 setup is overkill for any single monitor setup, I think we can agree on that. Lets say a year from now they start to really harness that power and test the cards. Yes, 3 years from now you wont be running at max settings and res, but it will surely be acceptable for the next 2 years minimum, and the couple years depends on your willingness to lower your settings a bit.
Look at the card released 3 years ago: 8800 ultra. Look at what amounts to buying 4 of those (as this is ~4 5850's or ~3 5870's.) Im not looking at any benchmarks but I can assure you those cards would still run current games just fine.
I understand what you're saying, but you can't argue with the fact that buying 2 of these now will set you up for the next few years unless you have ~$1500 to drop on a set of new cards every 12 months or so.