Splitter,
Its not always possible to use a full allocation of rest time to sleep. Most company's (certainly mine) roster more than their fair share of 24 hour slip patterns. If you could tell me how to fit 2 full sleep patterns into 24 hours I would be pleased to hear from you. Also, bear in mind that you usually arrive at your dest airport absolutely knackered so the option of staying up and having only 1 sleep is a non starter.
We all do our best, but sometimes (more often than not) - it impossible.
Usually, its people with no long haul experience who go on about this as they have never experienced the situation, and until you have....well, you just haven't!