Inconvenience aside, there is still a case for some charge to riders. If I take the subway and you stay home, I benefit a bit more and create a cost a bit more than you.
One could argue that the need to redensify cities is sufficiently compelling as to justify free public transit. There's more on this in the Persky/Wiewel chapter of my "End of Welfare?" book (M.E. Sharpe, 2000).
mbs
. . . Fare recovery is a lousy part of the transit world: I can't tell you how many green light cycles I've missed while waiting to board an SF Muni bus, or how many hundreds of hours I've spent waiting to pay my $2 at the Bay Bridge.
/jordan