There is a massive difference in text sharpness when comparing a rMBP 15" (900p in 2x) screen vs. 1080p 24" monitor, there are absolutely no doubts about this.

There is a massive difference in text sharpness when comparing a rMBP 15" (900p in 2x) screen vs. 1080p 24" monitor, there are absolutely no doubts about this. If you have perfect eyesight or exceed it, you would be affected less than the average users who will notice the difference. Are you sure you're not referring to the usable workspace? The usable workspace is half the native resolution when using the default 2x setting on OS X, in your case it is 1440x900 resolution or 900p on your 15" rMBP. The text should be very easy for your eyes to read for several hours compared to your 24" screen. For me, as I use the computer for more than 8-10 hours every day for work, Retina/Hi-DPI has helped with tired eyes at the end of the day. Eyestrain issues are like nonexistant with retina displays for me.