Daniel Stone recently wrote this amusing summary of why saying OMG I ABSOLUTELY MUST HAVE MY DPI CORRECT is rarely, if ever, correct.
Since most people who say this aren’t considering the two monitor case, I thought I’d provide a illustrative screenshot.
This is an old screenshot of the resolution-independence branch of GTK+ running the demo program on two different monitors. It shows that both apps the physical same size, with the same physical size fonts (ignore the size of the title bars, those are provided by Metacity, which is using the system GTK+).
However, the font rendering looks different. This is because of the amount of anti-aliasing that goes into making fonts look smooth when you have less pixels to play with. Notice how in the title bars the font looks the same, even though they’re different sizes. That’s because they’re using the same number of pixels, and thus the same amount of anti-aliasing.
I couldn’t find a screenshot of what happens when you have a window that crosses between two monitors. [Perhaps it's a nonsense case, but it can come up.] At this point everything just looks like crap. We have to choose the scaling of one monitor or the other. It will look wrong on one monitor or the other. It will be a different size to everything else on that monitor. [Before you suggest it; we can't divide the window and calculate the scaling separately for each monitor across single widgets].
The thing is, for day to day computing, you don’t really care about the DPI, you care about having readable, attractive fonts and visible, clickable icons. So Federico is right. For some specific applications, you do care about DPI, but those applications can already use the XRandR data to take care of themselves [I have written visualisation apps which do this, presenting a scale on the screen of millimetres on screen to metres in the real world.]
This said, I’m not sure why the only font size settings now in GNOME is a quantised Small, Normal, Large, Larger setting in the Universal Access settings pane. gnome-tweak-tool includes a scaling slider. The slider is hard to use though, especially when your text reflows while using it. Federico’s idea seems better.
As DPI continues to increase, it’s probably worth also applying scaling factors to our scalable window graphics, lest icons and the like become increasingly harder to hit. At this point we will likely need to adopt a resolution independent GTK+. However, again what will matter will be readable fonts and visible icons. Using the actual monitor DPIs will likely not be that important.