I'm with sacrifice333 in that universal tolling, it seems to me, would enable the government to adjust rates according to levels of congestion in certain areas at certain times of the day.
As for leaving at least one free route, wouldn't it make more sense to have negligibly low fares on less-used routes? 50 cents or a dollar wouldn't deter anybody if it was significantly less than the cost of using other routes, but would provide a source of revenue that pays for itself time and again. If people got the impression that these revenues are building better, safer bridges, surely that would make a difference.
Leaving one route free would strike me as politics, pure and simple. The Libs know if they toll all the bridges, on top of the carbon tax, that the (somewhat less green than they used to be) NDP will be all over that. A free route would not only miss out on valuable revenue at minimal user expense, but (and here I disagree with those who take a minimal level of average intelligence for granted) I'm sure that many people (many of whom already have the mistaken impression that transit is a horrible way to go) would go significantly out of their way, spend an extra 20 to 30 minutes driving, an extra $5-10 on gas, etc., if they thought they were somehow getting one back by not paying the toll.
Let's also consider the additional cost to the environment (in terms of natural beauty [isn't the air quality there already bad enough?], but also land values in surrounding municipalities) of a congested Alex Fraser!
I agree with paradigm4's suggestion that the possibility of leaving the Patullo as a greenway should be looked into (though I somehow cannot see it working with a bridge that size, given the paucity of cyclists currently living in the Scott Road area, and the lack of those living in New West and wanting to cycle to that part of Surrey). Needless to say, any conversion of the bridge to pedestrian/cycling traffic would involve significant infrastructure changes on both sides of the river - not just approaches, etc., but in Surrey, the issue of land use as well. It's not a process, I think, that would be financially competitive with tearing the old bridge down (though I agree that that would be a shame). Having said that, Vancouver and Greater Vancouver have spawned some pretty innovative ideas - A brainstorm on what to do with the old bridge would likely yield some interesting suggestions!