Having done environmental work in the past the idea seems completely viable. The greatest amount of pollution around an airfield is from ground based operations, taxiing, road vehicles etc. It will have a beneficial impact. How that stacks up with having sufficient 'push back' staff/tugs and the cost of those too, to implement it somewhere like Heathrow remains to be seen. Another issue is in LVP's, it will make it very interesting for ATC to resolve those issues if it became standard practise. I imagine it would quickly revert to powered operations to become manageable.
The other benefit no one seems to have latched onto is the potential reduction of noise too. It would make a considerable contribution to the local enviroment to cut out a significant amount of the taxiing noise.