As nice at it might be to get perfect framing in every shot, it's all but impossible, even with SLRs. At the taking stage, few cameras have absolutely 100% viewfinders and even fewer print labs print the full image, unless the image is reduced in size while leaving the canvas at their standard print size, leaving a border. Of course, you could be a home printer and so have the complete control (other than the potential gamut problems) that allows.
Having grown up got older with film and lab printing (as well as doing a fair bit of D&P myself in B&W days), I'm used to effectively leaving a little space round a subject knowing that the 'border' will be cropped by the lab (or me at PP stage), or in the case of a non 100% VF, knowing that there's a little extra to play with that I can't see through the VF.
There is another benefit of using the screen - the extra info it holds over that supplied by the X-20's OVF overlay (or even none from the X-10 - my main reason for upgrading to the X-20). In my case, the artificial horizon is invaluable since I have real problems keeping the sea from flowing out of the frame without it!
Of course, traditionally the photographer HAD to use a screen on the back of the camera - thankfully, these days that screen is bright enough to see without a black sheet covering us and the camera back!
