did a quick infographics for ya tNuvolari
pls ignore the omitted details on the headset part and etc. tried to depict simple comparison between the use of SPACER VS QUILL ADAPTOR on threadless setup. the 'dashed line' is where the steerer tube is cut.



this pic is what i've tried with spacers once for my SS clunker. (1" steerer tube to 1" stem, the inside diameter of the spacer was hence 1").
as you see, there's one problem. where the expanding bolt is deployed inside the tube was way to far from the idealistic tightening point—right inside the top part of the headset. however, i didn't encounter a big trouble. maybe i was lucky not having had any front collision.