Keepcups are about 300 grams - minus the wieght of a disposable cup so its the fuel burn for carrying 280 grams for the one person. Go ahead and do the sums if you want, I'm sure they'll so the environmental impact of that fuel burn plus manufacturing the keepcup is less than the impact of throwning out 125 tonnes of disposable cups a day.... and then take it on board with you, there and back. Wonder what the extra fuel burn will be (over multiple pax, multiple flights) and how green that will be?
Considering my wieght can change upto 3 kg day to day, 280 grams is nothing...