Think of it this way:
Do you want some schmuck making $7.25 an hour (or whatever the minimum wage is) driving around a big 40,000 pound, 40 foot long brick on wheels?
Anyone theoretically could drive a bus. Turn the ignition, pop in the air brake valve and hit the gas pedal.
But, can that untrained schmuck off the street drive the bus safely and on route, on schedule, while NOT getting into any collisions or breaking laws, taking care of the passengers (yes, transit bus drivers do have to wear the customer service hat - they have to take fares, issue transfers, and assist customers with where they are going)...?
The real question - one that has been skirted around by most everyone - is why is TriMet's benefit package so expensive. The union doesn't buy the insurance, TriMet does. The union simply agrees to a benefits package that states what benefits will be offered. That's it. TriMet agreed to it. It's a no-brainer that health insurance for a bus driver is going to be more expensive than some Metro pencil-pusher - you're outside, in traffic, breathing God knows what, dealing with some passengers who treat you like garbage. Your risk profile is higher.
But no one, no where, has actually compared the cost of TriMet's benefits package to similar transit agencies elsewhere. How does TriMet stack up to, say, the Utah Transit Authority, or Denver's RTD, or Seattle's Metro Transit? Nobody knows, and nobody has investigated it. All we know is that TriMet's health insurance is more expensive than Metro. Well, duh! It isn't hard to insure pencil-pushers who sit behind a desk all day and are encouraged to ride their bike to/from work (even if it means they only work from 11:00 AM until 3:00 PM for a "full-time" shift.) TriMet drivers are all but required to drive to work (it says so in the driver's application when the position is offered) because drivers must be able to report for work in the early morning hours when there is no public transportation available and it's a really bad idea to be on a bike at that time of day. As an office worker I have a nice luxury of having access to a gym (no I do not work for TriMet) but a Bus Driver isn't always returned to Center Street Garage on their lunch time to use TriMet's gym facility.
The bottom line is that you get what you pay for. If TriMet drivers aren't worth $20/hour with benefits - fine. We'll see a lot more bus-versus-pedestrian incidents, a lot more collisions, a lot more unsafe situations, a lot more "bus went the wrong way" situations... It is not unlike the difference between the mainline airlines and the regionals - the ones whose pilots often make less than $35k/year and are often reported flying to the wrong airport, causing the runway incursions, crashing... You don't hear about too many 747 or 777 captains making those mistakes.