A 12pt font should look useful everywhere
In the past I used to argue that accurate desktop DPI was important, since after all otherwise you could print out a 12 point font and hold up the sheet of paper to your monitor and it would be different. I later gave up on the idea that accurate DPI measurement was generally useful to the desktop, but there's still something seductively attractive about the idea that a 12 point font should be the same everywhere, and someone recently wrote about that. Like many seductively attractive ideas it's actually just trying to drug you and steal your kidneys, so here's why it's a bad plan.
Solutions solve problems. Before we judge whether a solution is worthwhile or not, we need to examine the problem that it's solving. "My 12pt font looks different depending on the DPI of my display" isn't a statement of a problem, it's a statement of fact. How do we turn it into a problem? "My 12pt font looks different depending on which display I'm using and so it's hard to judge what it'll look like on paper" is a problem. Way back in prehistory when Apple launched the original Mac, they defined 72dpi as their desktop standard because the original screen was approximately 72dpi. Write something on a Mac, print it and hold the paper up to the monitor and the text would be the same size. This sounds great! Except that (1) this is informative only if your monitor is the same distance away as your reader will be from the paper, and (2) even in the classic Mac period of the late 80s and early 90s, Mac monitors varied between 66 and 80dpi and so this was already untrue. It turns out that this isn't a real problem. It's arguably useful for designers to be able to scale their screen contents to match a sheet of paper the same distance away, but that's still not how they'll do most of their work. And it's certainly not an argument for enforcing this on the rest of the UI.
So "It'll look different on the screen and paper" isn't really a problem, and so that's not what this is a solution for. Let's find a different problem. Here's one - "I designed a UI that works fine on 100DPI displays but is almost unusable on 200DPI displays". This problem is a much better one to solve, because it actually affects real people rather than the dying breed who have to care about what things look like when turned into ink pasted onto bits of dead tree. And it sounds kind of like designing UIs to be resolution independent would be a great solution to this. Instead of drawing a rectangle that's 100 pixels wide, let me draw one that's one inch wide. That way it'll look identical on 100dpi and 200dpi systems, and now I can celebrate with three lines of coke and wake up with $5,000 of coffee table made out of recycled cinema posters or whatever it is that designers do these days. A huge pile of money from Google will be turning up any day now.
Stop. Do not believe this.
Websites have been the new hotness for a while now, so let's apply this to them. Let's imagine a world in which the New York Times produced an electronic version in the form of a website, and let's imagine that people viewed this website on both desktops and phones. In this world the website indicates that content should be displayed in a 12pt font, and that both the desktop and phone software stacks render this to an identical size, and as such the site would look identical if the desktop's monitor and the phone were the same distance away from me.
The flaw in this should be obvious. If I'm reading something on my phone then the screen is a great deal closer to me than my desktop's monitor usually is. If the fonts are rendered identically on both then the text on my phone will seem unnecessarily large and I won't get to see as much content. I'll end up zooming out and now your UI is smaller than you expected it to be, and if your design philosophy was based on the assumption that the UI would be identical on all displays then there's probably now a bunch of interface that's too small for me to interact with. Congratulations. I hate you.
So "Let's make our fonts the same size everywhere" doesn't solve the problem, because you still need to be aware of how different devices types are used differently and ensure that your UI works on all of them. But hey, we could special case that - let's have different device classes and provide different default font sizes for each of them. We'll render in 12pt on desktops and 7pt on phones. Happy now?
Not really, because it still makes this basic assumption that people want their UI to look identical across different machines with different DPI. Some people do buy high-DPI devices because they want their fonts to look nicer, and the approach Apple have taken with the Retina Macbook Pro is clearly designed to cater to that group. But other people buy high-DPI devices because they want to be able to use smaller fonts and still have them be legible, and they'll get annoyed if all their applications just make the UI larger to compensate for their increased DPI. And let's not forget the problem of wildly differing displays on the same hardware. If I have a window displaying a 12pt font on my internal display and then drag that window to an attached projector, what size should that font be? If you say 12pt then I really hope that this is an accurate representation of your life, because I suspect most people have trouble reading a screen of 12pt text from the back of an auditorium.
That covers why I think this approach is wrong. But how about why it's dangerous? Once you start designing with the knowledge that your UI will look the same everywhere, you start enforcing that assumption in your design. 12pt text will look the same everywhere, so there's no need to support other font sizes. And just like that you've set us even further back in terms of accessibility support, because anyone who actually needs to make the text bigger only gets to do so if they also make all of your other UI elements bigger. Full marks. Dismissed.
The only problem "A 12pt font should be the same everywhere" solves is "A 12pt font isn't always the same size". The problem people assume it solves is "It's difficult to design a UI that is appropriate regardless of display DPI", and it really doesn't. Computers aren't sheets of paper, and a real solution to the DPI problem needs to be based on concepts more advanced than one dating back to the invention of the printing press. Address the problem, not your assumption of what the problem is.