Horses for courses. One is a varmint rifle that CAN be pretty good on small up to medium deer...the 243, the other is a deer rifle that can be pretty good on varmints.
The ballistic charts will tell you, however, that the 25-06 doesn't actually do anything much more with a 100 grain bullet that the 6mm Remington or 243 Winchester don't already do adequately and that the 25-06 doesn't do anything with the 117 grain bullet that the 270 doesn't do already adequately with the 130 grain bullet.
But as said on the American forums and etc., someone somewhere must think that it is a better ONE RIFLE "all around" cartridge for their particular purpose than having a two rifles a 243/6mm and a 270 WCF.
Given that here in the UK we don't "do" varmint shooting as the Americans do I suppose that the 25-06 gives that little bit extra performance with its 117 grain bullet that does the 243/6mm with its 100 grain bullet on deer.
But I bet that the makers of powder love it...as others say it is a powder burner that is for sure!