Have you tried using it? If set a scale of one to ten, with one the quality of the MacBook Air's non-Retina display and ten the quality of the rMBP at the "best for Retina" setting, then any of the other four HiDPI settings available in System Preferences have a quality of nine.
As a customer, I would be happy if Apple were to increase the resolution on the 15" MBP to 3840x2400 raw (1920x1200 HiDPI) because it would mean I could have an iota sharper screen. However, as an Apple shareholder, I would be unhappy about it because it would increase Apple's costs for a benefit that I think most customers would not notice.