Alright... that seems good... maybe. But suppose someone manually sets a parent's context to something other than the default - and it turns out not to be the one that is most intuitive or useful when all of the children are complete? Isn't the parent then at risk of "falling through the cracks"?
In other words: Some users may want certain parents to show up in a particular context when the children are complete - if they tend to plow through a perspective which shows a single context. Misjudging what context should be applied may result in a stalled project, and some difficulty in tracking down the culprit. A hard-wired parents bin (which displays and counts parents according to the applied filters) would help here.
Keep in mind that pre-1.8, projects don't appear in context view at all, meaning that they will effectively "fall through the cracks" anyway unless you pick them up as part of your weekly/daily review, which is done in planning mode. Further, I don't think there's any proposal that has the "Auto-complete" going away in 1.8, so projects that don't need to be reviewed manually upon completion can still be set to be completed automatically.