Bug 310258 added 'Show completed tasks' to Lightning UI but the option is 'Hide completed tasks' in Sunbird. As I was told 'Show completed tasks' is the right way for this option, so Sunbird should use the same label and feature. Putting a 0.9 Target Milestone as suggested by mvl on IRC.
After the checkin for Bug 372830 Lightnings now uses 'Hide completed tasks' too. In my opinion the change in this bug is therefore no longer required.
Fixed the other way around by the checkin for bug 372830. Please file a new post-0.7 bug if you think that we should switch the logic by 180 degrees.