I gave my highly qualified "depends on what your definition of is is" answer. Here's another response or a different reason.
An application engineer at Gosiger told me this: If you use dual contact holders, no debris or chips can get into the gap between the flange and the spindle face when it is in the spindle. If you use all dual contact holders this area stays clean. If you introduce just one or more gapped holders, chips and debris could accumulate when those tools are in the spindle and the when the next dual contact is indexed in you have mis-load. Conversely, if you used all gapped holders this debris will likely stay where it is and the subsequent tools would not mis-load.
The answer? Don't mix dual contact and conventional gapped holders in a dual contact spindle machine. If you have no choice, consider gluing in gasket material to close that gap. It won't improve anything stiffness-wise, but will keep the crap out.
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