I think, from my perspective as a scheme user who not resident in the Great Wen and who travels to London for work 5 or 6 times a month, they get people off the vastly overcrowded tubes more than anything else. I've ridden them in the rain, wearing clothes I'd normally walk/bus/tube in. In true downpours I bus it. Cabs are expensive and often not very fast.
The tube just means getting far to up close and personal with people whilst being broiled alive these days