I take your points.
Personally I would never accept a line up clearance unless the pre-take off checks were complete - and that includes the cabin of course.
On the same issue, many take off clearances at LGW this week seem to have included reference to landing traffic ie: Clear Take off, landing traffic 3 miles. Same message with different words perhaps?
Seems like a bit of overkill to me. I can only move a 737 so fast.