My original post referred to long haul flying. Over the sectors I fly, half the extra fuel would be burnt carrying it, meaning having to load say 10 tonnes extra to arrive with 5,000kg (lasting 30 mins on my type).
With many long haul departures up to MTOW, that means we'd need to leave 100 pax or 10 tonnes of freight behind just to carry fuel "just in case". Commercial madness, as I said.