Let me start off by saying I was one of the first guys who jumped in wholeheartedly and supported Godscountry's idea of a .264 Weatherby Magnum. I also proposed that it be based off the .300 Weatherby Magnum case and did so because I believed if it wasn't the .264 Weatherby Magnum would repeat the history of the .264 Winchester Magnum. Several people suggested that it be based on the .257, .270, or 7mm Weatherby Magnum cases which in reality is a .300 Weatherby Magnum case that has been shortened and necked down. Doing that would put it in the shadows of the 7mm Weatherby Magnum as this is the case with the .264 Winchester Magnum and the 7mm Remington Magnum since both of these cases are extremely similar when you compare each dimension between them. These dimensional similarities would also carry over to the .264 Weatherby Magnum when comparing it to the 7mm Weatherby Magnum so for me it was logical to go with the .300 Weatherby Magnum case to prevent this from occurring again and obtaining every bit of velocity possible.
I have a .264 Winchester Magnum and like it a lot. I always wanted one and when I saw the rifle, a Ruger M77 with the tang safety, I bought it and never gave it a second thought. It's very accurate, I've used it to take a lot of white-tailed deer, and never had to shoot any of them a second time. Regarding your comment that the 6.5 caliber not being a giant killer you're right, they're not. But they are right at home out in the planes and mountains of the West where the shots are long and open and the same will hold true with the 6.5-300 Weatherby Magnum. This was one of the reasons Winchester called their model 70s that were chambered the .264 Winchester Magnum the Westerner.
As for if we really needed a 6.5-300 Weatherby Magnum not really but there's the want factor. Oh yes the want factor, that is the proverbial ace in the hole so to speak. Many many times over want has trumped need and will continue to do so until the end of time. Let's be realistic here, do I really need my .338-378 Weatherby Magnum to kill a white-tailed deer or black bear in Pennsylvania? Hell no, never did and never will but I wanted it from the moment I first say the Accumark and read the ballistics on it. The same holds true for my 8mm Remington Magnum, .375 H&H Magnum, and .458 Winchester Magnum all of which I've shot white-tailed deer with, there's never going to be a day on God's green earth when I needed to use any of them to kill a deer however I wanted to. Crazy? I'm sure there's a lot of people out there who would say absolutely but it's what I wanted to do. No matter what caliber you're using you have to exercise proper shot placement or you'll end up having to fire multiple shots. The 6.5 or .264, which ever you prefer to call it by, caliber has been gaining a steady following in the past few years. People are starting to realize that it's a decent caliber that in many cases is inherently accurate especially with the variety of bullets that are available today and are using it more and more often. There's people out there that are going to buy a 6.5-300 for no reason other than it's a Weatherby where as there's others that see the true potential of it not only for hunting but for long range shooting and even target shooting. There's also a number of people who've become bored with the stagnant line up of calibers. For me I want one because I like the 6.5mm caliber, I see the potential of the round, and I had a hand in supporting the idea which has now become a reality. As for the need part, I need one to complete my Weatherby collection. Oh one other thing, the 6.5-300 Weatherby Magnum sounds really cool and impressive if you ask me.