Originally Posted by
HillRider
The uncut steerer (typical length 300 mm) is provided so the fork can be used on large frames with long head tubes, not specifically to let you put a huge spacer stack under the stem. A 62 cm frame could have a 220 mm headtube so allowing for a 30 mm headset stack height and 40 mm for the stem, that leaves only 10 mm of room for spacers.
One big advantage of threadless steerer tubes is that fork manufacturers need only make one fork with a steerer tube long enough such that it fits all reasonably sized frames. with threaded steerer tubes, the tubes had to be threaded to the appropriate point for the frame size (to avoid lots of unnecessary threads, a hassle for manufacturing) so many fork sizes were needed. With threadless, the excess length is simply cut off and discarded.