Yes, you are missing something. Of course, it could just my lack of communications skills. Note that I put quotation marks on either side if the word 'style.' It is not capitalized, I did not say it was a label for anything. I simply stated that the list of items in that left-hand panel shows the various "styles" of screen saver methods/displays; the way the images, no matter where they come from, are displayed and change on the screen. Thus the "style" of the screen savers. Just click on one and you can preview that style in the larger area to the right of the list. Just click and watch. This ia a Mac, you won't break it by clicking the buttons/icons/menus.
OK, this may take a bit longer, I can't get the add an image controls to appear, I'll upload the examples to DropBox and point to them. That'll save TS disk space, anyway.
OK, here is what I see when clicking the "Source:" label:
[attachment=2929:Desktop_...n_window.jpg]
Now, selecting the "Photo library..." item gives me the following:
[attachment=2930:Desktop_...ce_sheet.jpg]
So you can plainly see that access to your iPhoto images is still there. The "Shuffle slide order" replaces the "Random" choice for the selected images. And there is a 'Random' "style" available that will cycle through all the built-in screen saver styles with the images you have selected.
Bottom line, what more can you want in a useless function for LCD monitors?
OK, ok!! This is starting to get interesting (if somewhat maddening).....
It occurs to me that the difference may be that I a) don't have Aperture and b) I am using iPhoto '09.