[edit: added advertising]
[edit: added advertising]
As a case study: Transport for London have a very thorough advertising suite on buses and underground trains (and others).
Advertising and rent contribute £179m revenue. Ticket sales contribute £3533m revenue. Government grants contribute £3438m revenue.
So, a full suite of advertising may save almost 5% on ticket prices, as it stands.
Figures taken from TfL annual report 2012, relevant figures start page 36: http://www.tfl.gov.uk/corporate/about-tfl/investorrelations/...
The government is a paid service of the tax payer, no more holy than paying someone to mow your lawn. The sooner we realize the retroact of free is an illusion, the better. We need to pay for the infrastructure.
Them and the entirety of the Libertarian and Tea Parties, not to mention whatever Anarchists still exist.
Government (including, inter alia, the provision of public transit) is a paid service to the tax payer, but that's not the service we are talking about. We're talking about delivering transit rides to commuters, which is currently also in most places a paid service to the commuter, and the suggestion is to change it to a free service to the commuter. (Largely on the basis that making transit rides a paid service to the commuter is undermining the goal of government provision of public transit as a paid service to the taxpayer by discouraging utilization.)
It's more complex than that. If you take journeys that aren't necessary or use transportation in otherwise inefficient manners - ship flowers from halfway around the world in an aeroplane for example - then that's to everyone's detriment as it wastes non-renewable resources.
Everyone benefits from transportation to some extent but that certainly doesn't mean that encouraging more of it increases the benefit.