And I guess I should have explicitly stated that I'm not talking about high-DPI displays, subpixel rendering obviously doesn't do much good there!
My point is simply this, if you don't treat pixel like discrete little boxes that display a single color, you can use subpixels to effectively increase the resolution on low-DPI monitors. Yes, you can use greyscale antialiasing instead, you will even get better performance, but the visual quality will suffer on your standard desktop PC monitor.