Okay, so let's say I'm a device hacker. I have $mysterious_device. I want to get rid of $crappy_os and put something nice, open, and hackable on there. I take Mer, I create a "hardware adaptation" repository for $mysterious_device, then I create an image pulling in both Mer and packages for hardware adaptation, and if all goes well, I have an image that will boot, start a display, and not do much else. I decide that having a black screen is all very interesting but I'd actually like to put something on it, so then I go pick a UX project (MeeGo CE or Plasma Active or Cordia or something else), and drop them into my image building configuration, build a new image, if all goes well - when I install it - I have an image that boots to display and then starts that UX.