What do you mean picture quality? Unlike the HiDPI modes (the mode that makes things look larger on high pixel density displays) applications actually work correctly . If you are talking about actual pictures then the quality will be just as good or better on the non HiDPI mode.
If an app does not support HiDPI mode then it gets its resolution dropped to 1/4th and scaled up making it look bad. This does not happen on the non-HiDPI mode but instead its smaller.
Some things are rendered or can be sized up in the app itself which will make them look far better than the HiDPI mode (only apps with lack of HiDPI support).
For example in microsoft office or acrobat you can have it increase text sizes and what not when running non HiDPI mode and get the text size at the size you want and of retina quality. With images its pretty dependent on the resolution of the image. If its a high resolution image then it will be the same or better in an app that does not support HiDPI mode.
Rendered things will for sure be better in non HiDPI mode if the app does not support it.
Here is an example. In HiDPI mode go to maps.google.com and enable google earth inside safari. Look at how crappy google earth looks. Now do the same at the native 2880x1800 (non HiDPI mode) and it looks awesome. Even though safari supports HiDPI google earth does not so your wasting the display on it.
Well I am talking about for example Chrome at 150% zoom with 2880x1800 vs Chrome at 75% zoom with best for retina. Or other apps where the fonts are adjusted appropriately according to readability. Which one looks better? So pictures will look better on native 2880x1800
What about performance and battery life. If it does not need to scale when using native 2880x1800 then it does not need to waste resources on calculations. Thus increasing battery life and performance of the UI. Am I correct to assume that?