Of course, you could also ask why it should be necessary to place cuts on keyframes at all...............

As I understand it, cutting on keyframes means that practically no recoding is required, if not on a keyframe, it has to do more work on either side of the cut.
The problem with this, is that some channels seem to be pretty miserly with their keyframes. With the attendant possible missing out of dialogue/action.

Bruce S.

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