Wi-Fi (not) on the line?

So thanks to the fact that lately Twitter has been my major source of info anything-related, it's come to my attention that Transport for London are planning to introduce Wi-Fi services on the London Underground. All well and good, thanks very much. But, given that they're not going to be introducing it on the actual trains, does beg the question - why? If you're using the tube at rush hour, it's never more than 2-3min wait for the next train. How much googling/tweeting does one expect to do in that time..?

However, I acknowledge that it would be especially beneficial to have an outlet for commuters' anger when tubes are delayed/evacuated/too packed.

Though I'd also imagine that pretty soon they'd have to mock up some TfL advisory posters, along the lines of "I will not unwittingly hit into people in stations and on platforms while I've my nose buried in my Wi-Fi enabled device."

(Photo found here - thank you!)