It wouldn't save the planet but the saving can be quite substantial.
My airline operates 1400 flights a day, a 2 minute saving on each one of them would save approximately 1400*2*40(40kg a minute on a 737)=112000kg of fuel.
It's worth trying!
I do not understand why we even have to ask for directs?
If it's available it should be offered especially if the capacity is there to coordinate it with other atc providers.