There's no logical reason to double the resolution on Macs this way because they're resolution independent

Yes, but if they keep the current assets, the physical size of the interface will keep getting smaller as resolution goes up. To counter-act this, you have to increase the resolution of the assets as well. Non-integral scaling will invariably be janky and either look terrible due to scaling artifacts on apps that haven't been updated or just break apps altogether (see: Windows hi-DPI modes). Thus, OS X's hi-DPI mode, which is precisely pixel-doubled, just like Retina mode in iOS. Of course, if you enable hi-DPI mode on a screen that isn't at least quadruple the resolution of current Mac models, you lose physical screen space. The only way for them to utilize the hi-DPI mode introduced in Lion without reducing the amount of physical screen space available to the user is to use displays that are at least quadruple resolution - just like Retina displays. Which yes, may be expensive and may not exist yet, but hey - those Retina iPad displays didn't exist either until Apple placed their order.