It's like, why go 580 when you can OC 570 and get just as well?
The fact that it's free performance, easy and has no consequences (given you don't flash both bios), why not?
Besides, the twin frozr II is built to OC, but it's more expensive than regular 6950s at around $40~$50 price gap, there must be at least some difference, right?
In addition, the large RAM size helps in any resolution higher than 2560, going for the 1GB version might only be good for 1080p or 2560 res.