you will burn 10% of your fule just carrying it. so 10% of your extra fuel will be burnt away just for carrying it. we use the rule of thumb that the fuel price needs to be 10% less in order to tanker. our company does this whenever possible. now those who take extra fuel just to make themselves feel better burn away the money saved by those who tanker. so i don't think we ever save money do to others wasting a lot of fuel be it by carrying extra when not needed or flying technique such as decending early and not doing idle decents.