Despite the cynicism of some - and the flat-earther-stylee climate-change denial of others! - this issue is one that has to be tackled, and someone has to be first to do it. There is no denying that any airline that acquires a mantle of environmental awareness will have an advantage in a marketplace that is sensitive to such matters, however, and Branson is a past master of exploiting such things.
The tug-to-the-runway approach is a coarse first step in a process which will eventually see aircraft making their way to the runway with their main engines shut down. Whether that's by the 'shopping trolly' system mentioned earlier, or auxiliary electric axle motors on board, or some other system is irrelevent as yet - the fact is that the experiment and development phases must begin somewhere. I'm sure most of us can see many of the disadvantages in using old, dirty, and scarce tugs to drag aircraft to somewhere near the pre-departure hold, but this is surely to establish simply how much fuel can be saved and what problems actually transpire, as opposed to what might be anticipated. From the data and experiences learned, decisions can be made as to what the next step should be.