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I believe the idea is that the magic bung braces the steerer. That top cap thing is a tight fit in the steerer and shares the clamping force of the stem. If you use spacers above the stem it partially loses it's ability to brace, depending on how many you use. I think - but I've no proof - the steerers on the 'dale forks are designed to be used with this type of bung, as a 'system', hence SI, and not using that type of bung could result in a fiery death, or something.
(there's a technote that goes into this somewhere, but I can't find it).